Link to source code file link can now point to another base directory

I added the possibility to prefix the target file that will be linked on the error page.
This is useful if you are using a vagrant box for your rails app and the path on your box differ from the path in your local repository.
Simply set the OS environment variable BETTER_ERRORS_FILENAME_PREFIX (starts and ends with a slash, e.g. '/Users/kobe/Development/railsapp/').
I have chosen an environment variable because at my company we work many people on the same project and configuring one path in the rails app would not work out.