Improve checking of markdown extensions

Previously Markdown extensions used symbol to string conversion to check
for markdown extensions.

Now Markdown extensions are checked by using the object from the closure
which is cleaner and creates fewer garbage objects.
Reduce string matching to improve performance

Matching strings in the grammar results in increased method calls which
decreases the speed of the parser.

Expanded use of regular expressions decreased parse time by around 20%
Use regexp to match Markdown NewLine and Spacechar

Like the previous commit, regular expressions speeds up markdown parsing
for ruby 1.8.
Replace kpeg repetition with regular expressions

This slightly improves parsing speed as regexp repetition is faster than
a while loop.