Revert "Revert "Merge pull request #695 from github/detect-prose""
Explicitly mention prose types within languages.yml
Add helper method to retrieve languages by type
Set Textile as a prose language
Include RST in the languages list
Alphabetize RST correctly
Cleanup languages file to fix tests
Merge pull request #819 from github/tell-me-a-story-of-prose

Explicitly mention prose types as `prose`