The smart_strong extension has been removed and its behavior is now the
default (smart em and smart strong are the default). The legacy_em
extension restores legacy behavior.

This completes the removal of keywords.  All parser behavior is now modified
by extensions, not by keywords on the Markdown class.

If you have existing documents that use the legacy attributes format,
then you should enable the legacy_attrs extension for those documents.
Everyone is encouraged to use the attr_list extension going forward.

The previous version did not work, and was incorrectly removed as part of
85ad18071d619251. In the new version, use lookbehind search for \w instead
of \b, so that it works.

Update the tests accordingly.

Fixes #399 (except parts that we can't fix). Thanks @gandaro for the report.

Fixes #389. Thanks for the report @lazka.

Fixes #380. The TOC replacement is now seperate from building the
TOC, which allows us to iter through the doc in order rather than
with the non-order-preserving interparent pethod. This is almost a
complete refactor of the run method. Also cleaned up the config stuff.

Thanks to @colewerner for the report and test case.

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.