Add reset support to TOC extension.

Now, whenever the TOC extensiuon is loaded, the Markdown class instance
will always have a toc attribute (md.toc). Calling md.reset() will also
reset the toc attribute which defaults to an empty string.
Fix test
Add reset support to TOC extension.

Now, whenever the TOC extensiuon is loaded, the Markdown class instance
will always have a toc attribute (md.toc). Calling md.reset() will also
reset the toc attribute which defaults to an empty string.
Always add toc to Markdown Class Instance

Previously, we only added the toc attribute (md.toc) if no Marker was
found within the document. However, that has caused framworks to do things
like force insert a marker, run convert, then extract the toc from the
body of the document. This is much cleaner. And if the user wants to add
the toc to the document also, they still can.
Complete test coverage of TOC Extension
Merge branch 'temp'

Conflicts:
	tests/test_extensions.py