Enable file exclusions in prompt

I often want to change the occurrence of a pattern that's relatively common in the codebase, but I only want to change it in certain places. When prompted for file exclusions, entering a single file by its name (e.g. `single_file.rb`) or multiple files separated by commas (e.g. `single_file.rb,other_file.rb`), will exclude the selected file(s). 

NOTE: File names are assumed to be unique (i.e. `lib/foo.rb` and `tmp/foo.rb` will both be ignored if `foo.rb` is given).