Issue #368: Fix Markdown in raw HTML stops working

Originally there was an infinite loop issue that was patched in issue
#308.  Unfortunately, it was fixed all the way.  This fix patches the
infinite loop fix to only add an offset to the `right_listindex` when
it is in a infinite loop scenario.

Fix issue #365 and #366

This reworks the toc ordering to be done in a single pass with no
recursion.  Very long documents with lots of headers can actually
exceed Python’s max recursion limit.  By handling the toc ordering with
no recursion, large documents can no longer cause toc to fail with
recursion erros.

The logic for the current regex for strong/em and em/strong was sound,
but the way it was implemented caused some unintended side effects.
Whether it is a quirk with regex in general or just with Python’s re
engine, I am not sure.  Put basically `(\*|_){3}`  causes issues with
nested bold/italic. So, allowing the group to be defined, and then
using the group number to specify the remaining sequential chars is a
better way that works more reliably `(\*|_)\2{2}.  Test from issue #365
was also added to check for this case in the future.

Fixes #361.

Fixes #359. Thanks for the report @gremmie.

Changed logging level when loading extensions

So it suppresses messages for loaded extensions when its used as third party.