Add official tests from daringfireball.net.

Also enable process isolation to temporarily work around a
memory issue with one of the tests.

Switch to $GLOBALS from constants to get process isolation
working.
Temporarily disable testMarkdownDocumentationSyntax() until out of memory error is fixed.
If no block is found, go to afterSpace state, instead of mol state.

This will help to avoid bugs when additional afterSpace rules are put
in place, but they aren't checked in startParagraph. Also helps to
avoid code duplication.
Group official tests by features tested and disable those with unimplemented features.
Start rewrite of tokenizer to handle nested blocks better.

Warning: Breaks a lot of tests.