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