Review use of globals for determining working directory, module loading, extension loading, etc

BeEF uses `$root_dir` and `$home_dir` global variables to define the BeEF root directory and user's home directory respectively.

https://github.com/beefproject/beef/blob/124c9d60b3cb4138816ebc9a301e98db86186307/beef#L38-L40

These globals are also referenced throughout the codebase.

```
# grep -rn "\$root_dir" core/ extensions/ modules/
core/main/configuration.rb:21: # by default loads $root_dir/config.yaml
core/main/configuration.rb:245: Dir.glob("#{$root_dir}/extensions/*/config.yaml") do |cf|
core/main/configuration.rb:252: y['beef']['extension'][y['beef']['extension'].keys.first]['path'] = cf.gsub(/config\.yaml/, '').gsub(%r{#{$root_dir}/}, '')
core/main/configuration.rb:263: module_configs = File.join("#{$root_dir}/modules/**", 'config.yaml')
core/main/configuration.rb:271: y['beef']['module'][y['beef']['module'].keys.first]['path'] = cf.gsub('config.yaml', '').gsub(%r{#{$root_dir}/}, '')
core/main/command.rb:232: component_path.replace "#{$root_dir}/core/main/client/#{component_path}.js"
core/main/network_stack/websocket/websocket.rb:32: cert_key = File.expand_path cert_key, $root_dir unless cert_key.start_with? '/'
core/main/network_stack/websocket/websocket.rb:39: cert = File.expand_path cert, $root_dir unless cert.start_with? '/'
core/main/server.rb:121: cert_key = File.expand_path cert_key, $root_dir unless cert_key.start_with? '/'
core/main/server.rb:128: cert = File.expand_path cert, $root_dir unless cert.start_with? '/'
core/main/handlers/modules/legacybeefjs.rb:19: beef_js_path = "#{$root_dir}/core/main/client/"
core/main/handlers/modules/legacybeefjs.rb:128: component_path.replace "#{$root_dir}/core/main/client/#{component_path}.js"
core/main/handlers/modules/beefjs.rb:19: beef_js_path = "#{$root_dir}/core/main/client/"
core/main/handlers/modules/beefjs.rb:128: component_path.replace "#{$root_dir}/core/main/client/#{component_path}.js"
core/main/autorun_engine/rule_loader.rb:84: Dir.glob("#{$root_dir}/arerules/enabled/**/*.json") do |rule|
core/module.rb:94: mod_path = "#{$root_dir}/#{config.get("#{mod_str}.path")}/module.rb"
core/extension.rb:37: if File.exist? "#{$root_dir}/extensions/#{ext}/extension.rb"
core/extension.rb:38: require "#{$root_dir}/extensions/#{ext}/extension.rb"
extensions/social_engineering/powershell/bind_powershell.rb:50: ps_payload_path = "#{$root_dir}/extensions/social_engineering/powershell/powershell_payload"
extensions/evasion/evasion.rb:36: unless File.exist?("#{$root_dir}/extensions/evasion/obfuscation/#{technique}.rb")
extensions/metasploit/api.rb:30: path = "#{$root_dir}/#{BeEF::Core::Configuration.instance.get('beef.extension.metasploit.path')}/msf-exploits.cache"
extensions/admin_ui/classes/httpcontroller.rb:131: template_ui = "#{$root_dir}/extensions/admin_ui/controllers/#{class_s}/#{function_name}.html"
extensions/admin_ui/api/handler.rb:127: Dir["#{$root_dir}/extensions/admin_ui/controllers/**/*.rb"].sort.each do |http_module|
modules/exploits/firephp/module.rb:16: f = File.open("#{$root_dir}/modules/exploits/firephp/payload.js")
modules/exploits/rfi_scanner/command.js:39: File.open("#{$root_dir}/modules/exploits/rfi_scanner/rfi.txt", 'r') do |file_handle|
modules/exploits/shell_shock_scanner/command.js:40: File.open("#{$root_dir}/modules/exploits/shell_shock_scanner/shocker-cgi_list", 'r') do |file_handle|
modules/social_engineering/firefox_extension_dropper/module.rb:32: mod_path = "#{$root_dir}/modules/social_engineering/firefox_extension_dropper"
modules/social_engineering/firefox_extension_dropper/module.rb:70: bind_extension.set :extension_path, "#{$root_dir}/modules/social_engineering/firefox_extension_dropper/extension/#{@xpi_name}.xpi"
modules/social_engineering/simple_hijacker/command.js:19: tplpath = "#{$root_dir}/modules/social_engineering/simple_hijacker/templates/#{@choosetmpl}.js"
modules/social_engineering/simple_hijacker/module.rb:18: tplpath = "#{$root_dir}/modules/social_engineering/simple_hijacker/templates/#{template}.js"
modules/social_engineering/firefox_extension_reverse_shell/module.rb:34: mod_path = "#{$root_dir}/modules/social_engineering/firefox_extension_reverse_shell"
modules/social_engineering/firefox_extension_reverse_shell/module.rb:63: bind_extension.set :extension_path, "#{$root_dir}/modules/social_engineering/firefox_extension_reverse_shell/extension/#{@xpi_name}.xpi"
modules/social_engineering/firefox_extension_bindshell/module.rb:33: mod_path = "#{$root_dir}/modules/social_engineering/firefox_extension_bindshell"
modules/social_engineering/firefox_extension_bindshell/module.rb:60: bind_extension.set :extension_path, "#{$root_dir}/modules/social_engineering/firefox_extension_bindshell/extension/#{@xpi_name}.xpi"
modules/social_engineering/ui_abuse_ie/module.rb:23: popunder = File.read("#{$root_dir}/modules/social_engineering/ui_abuse_ie/popunder.html")
modules/browser/fingerprint_browser/command.js:11: f = "#{$root_dir}/modules/browser/fingerprint_browser/fingerprint2.js"
```

```
# grep -rn "\$home_dir" core/ extensions/ modules/
core/logger.rb:15: @logger ||= Logger.new("#{$home_dir}/beef.log").tap do |log|
modules/host/get_wireless_keys/module.rb:15: filename = "#{$home_dir}/exported_wlan_profiles_#{ip}_-_#{timestamp}_#{@datastore['cid']}.xml"
modules/browser/spyder_eye/module.rb:28: filename = "#{$home_dir}/screenshot_#{ip}_-_#{timestamp}_#{@datastore['cid']}.png"
```

This approach is inconsistent. Investigate a cleaner approach. One option would be to build BeEF as a Ruby gem and using helpers which refer to the gem's root directory.