Add variable for file relative path

[dacook]

It would be awesome if we could have an option to copy the file path relative to the project path. This would make for a much smaller string which is easier to read, and manage when combining with other commands.

For example, this is way long:

rspec /Users/xxx/projects/xxx/spec/system/admin/import_spec.rb:48

But I'm always working from the project directory, so it would be great to have simply:

rspec spec/system/admin/import_spec.rb:48

This doesn't have to be the case by default. I suggest a new variable, like $relative_filepath, so that you can optionally add that to your config as you see fit.

[dacook]

I just realised I could implement this with regex capture groups in the config.
For example, assuming your projects are all direct children of a "projects" folder:

	  ["projects/[^\\/]+/(.*_spec\\.rb)$",  "rspec $1:$line_number"  ],

However this doesn't handle other project locations. (It could, if we had a variable for the project path.)