Recognize Thor files as Ruby files

[vitallium]

Hi. This pull request add Thorfile and *.thor files to the filenames of the Ruby lexer since they are Ruby files.

Here are before&after screenshots.

I used these two files: Thorfile and command.thor from the Rails repo to make them:

Command: ./bin/rougify <filename> --theme=github

Before	After

Reference:

• https://github.com/rails/thor/wiki/Getting-Started#file-names

Thanks!

[tancnle]

Thank you @vitallium. LGTM 🚀