Update language whitelist with Github's own list of languages

I went to suggest a project and, horror of horrors, it was impossible to submit because the mandatory "Main language" field did not recognize the language in which the project was written. Given that 24pullrequests leverages Github, it seems only proper that it should allow all the languages that Github itself acknowledges.

Language list copied out of the "Other: Languages" dropdown on https://github.com/trending

Closes #335.