Allow to configure internal url

[gucki]

Right now the internal path seems to be hardcoded to /__better_errors. See https://github.com/charliesome/better_errors/blob/master/lib/better_errors/middleware.rb#L74

It should be possible to configure/ prefix this path. This is needed if the rails project is not "mounted" at the root level, ex at /my_rails_app1/.... The correct path would then be /my_rails_app1/__better_errors

[gucki]

@RobinDaugherty Sorry, I don't really get it. Could you point me to the code you are referring to? In my setup it works without any problems so far...