Add Public Key as a language with .asc and .pub extensions

This pull request adds a `Public Key` language. It's not really a language but it would solve many misdetections:
- https://github.com/search?utf8=%E2%9C%93&q=extension%3Aasc+BEGIN+PUBLIC+KEY+BLOCK
- https://github.com/search?utf8=%E2%9C%93&q=extension%3Apub+BEGIN+PUBLIC+KEY+BLOCK&type=Code&ref=searchresults
- https://github.com/search?utf8=%E2%9C%93&q=extension%3Apub+ssh-rsa&type=Code&ref=searchresults

The samples currently contain two PGP and two SSH public keys. The Bayesian classifier seem to do a good job here but I'd like to make sure.
@arfon Could you grab me a corpus of files from the search results?