Add ability to monkey patch frontman / global_require

Its pretty difficult to monkey patch frontman. Currently I am using the following technique at the top of my config file.

```ruby
# config.rb

::Frontman::App.class_eval do
def import_config(path)
eval File.read(path)
end
end
```

It would be nice if this could be extracted to another file somehow. The problem here is that the context is within `Frontman::App`. Maybe we could have a method like `global_require` that requires files from the global context.

Start with config.rb and the Frontman::App#import_config override shown in the issue, then trace how configuration files are loaded and evaluated. Done means an extracted file can be loaded from the global context without requiring the inline monkey patch; the issue does not name a test to run.

Written by the indexing model from the issue text.