Add renderer feature. This is a updated version of #129.
At #129, it only contains block level renderers. This patch contains
both block level and span level renderers.
Since renderer feature can do many things, I removed highlight option
and headerPrefix option.

https://github.com/chjj/marked/pull/129
Fix table align. Marked use style text-align instead of the align
attribute.
Fix test cases. Since we have removed the headerPrefix option. There
will be no id on headings.
Remove documentation on highlight, since we won't need them.
Remove documentation on langPrefix.
Add documentation on renderer.
Merge remote-tracking branch 'lepture/feature-renderer'

Conflicts:
	README.md
	lib/marked.js
reimplement async highlighting feature. whitespace.