Suggestion for vagrant: Auto set 'trusted_ip'

I would like to suggest to maybe change the documentation to simplify the configuration for those working with vagrant.

I have simply created this file, and then you can start the server normally (without setting `TRUSTED_IP`)

``` ruby
# /config/initializers/better_errors.rb
if defined? BetterErrors
trusted_ip = ENV['SSH_CLIENT'].split(' ').first
puts "=> better_errors is open for #{trusted_ip}"
BetterErrors::Middleware.allow_ip! trusted_ip
end
```