Add a shell lexer to libphutil

Summary: I need this to properly parse SSH_ORIGINAL_COMMAND in the sshd receivers. We get a string which may have shell escaping.

Test Plan: Wrote a ton of unit tests.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T550, T603

Differential Revision: https://secure.phabricator.com/D4230