Add a simple language guesser for shells scripts and other things we can reasonably infer the language of

Summary:
If we can't figure out the language a file is written in from path/filename
rules, try to look for telltale markers in the file itself.

This provides a few very simple rules, but they should have no false positives
and cover most reasonable cases.

Test Plan:
Pasted files with no identifying filename into Paste, got reasonable
highlighting.

Reviewers: btrahan, bill, nh

Reviewed By: nh

CC: aran, epriestley

Maniphest Tasks: T513

Differential Revision: https://secure.phabricator.com/D1618