Merge pull request #1 from metacpp/master

Merge with the newest branch
Merge pull request #2 from metacpp/master

Merge the latest commit from Qingqing
Add two validation rules (preceding & following)

We need to validate "> [WACOM.NOTE [a](b)]".
And the preceding rule will validate string "> "; the syntax rule will
validate string " [a](b)"; and the following rule will validate the
string "" (because nothing follows the ']' character).
Merge pull request #6 from JunyiYi/master

Add "Preceding rule" and "Following rule" validation.