Drppoed the non-greedy quantifier from the end of the inlinePatterns as it
served no useful purpose and was actually (in very rare edge cases) causing
newlines to be dropped. FIxes #439. Thanks to @munificent for the report.

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.

Previously, instances of the Markdown class were represented as any one of
'md', 'md_instance', or 'markdown'. This inconsistancy made it diffcult
when developing extensions, or just maintaining the existing code.
Now, all instances are consistantly represented as 'md'.

Apparently this is a new requirement of flake8. That's the thing about using
tox. Every test run reinstalls all dependencies so an updated dependency might
instroduce new errors. I could specify a specific version, but I like staying
current.

Got all but a couple files in the tests (ran out of time today).
Apparently I have been using some bad form for years (although a few
things seemed to look better before the update). Anyway, conformant now.

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.