Adds method to allow all connections to see better errors

This pull request adds a new class method to the middleware class that specifies allowing all connections the ability to view Better Errors. Rather than whitelisting IPs for teams, this method can be called in the dev environment.rb file switching Better Errors on for all connections. Use case is using remote servers (i.e. AWS) as the dev and staging environments and allowing multiple people on a team access to the boxes. Whitelisting IPs can be cumbersome for large teams.