wip: inline & display math

Signed-off-by: Server <bkpk@valyria.backpack-books.local>
remove console.log

Signed-off-by: Server <bkpk@valyria.backpack-books.local>
use `$ ... $` for inline, `$$ ... $$` for display

test is also added

Signed-off-by: Server <bkpk@valyria.backpack-books.local>
math : refactor to renderer

Signed-off-by: Server <bkpk@valyria.backpack-books.local>
Merge branch 'mathjax'

Conflicts:
	lib/marked.js