[Markdown] Optimize paragraph patterns and reorganize variables (sublimehq#4230)

This PR...

1. optimizes paragraph termination patterns (1st commit) to avoid
   catastrophic backtracking caused by chaining whitespace patterns
   like `\s*(?:\s*...|\s*...)` (simplified).

   This commit improves parsing performance of
   syntax_test_markdown.markdown by about 6-7%

2. reorganizes variables by category and adds some separating comments.

Note: Beyond improving performance, this PR does not change behavior.

Author: deathaxe