Update albino_filetype.rb

Added 3 new pygments rules:

 * html (for HTML4 and XHTML 1, couldn't find a lexer for html 5)
 * po (translation files)
 * rst (markup language)

Tested and working fine on my production server after new deployment, flushing the redis cache and restarting it :)
Merge pull request #428 from Fandekasp/patch-3

Update albino_filetype.rb