document: Implement MATH extension
hoedown: Add new flags to CLI
html: Add *provisional* implementation for MATH
document: Make MATH extension context-sensitive
Rename MATH_DOLLAR to MATH_EXPLICIT, update descriptions
Keep up to date with master

Conflicts:
	src/document.c
Merge pull request #114 from jmendeth/mathjax-2

MathJax support, second attempt