Make `#` header detection more strict

Summary:
Ref T5102. Require a space after a single `#` at the beginning of a line to possibly trigger header behavior.

This prevents conflicts with `#projects` and some normal text beginning with `#`.

Test Plan: Added unit test.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: pieter, epriestley

Maniphest Tasks: T5102

Differential Revision: https://secure.phabricator.com/D10151