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.

Use the TOC extension instead.

There is no need for this anymore. In the past, most config was using
globals. Today all config is held on the class instance.

All previously supported methods of loading an extension given a string
name have now been replaced with entry points. Extension classes must be
registered as entry points in the 'markdown.extensions' group to support
string names. The old module-level 'makeExtension' function is no longer
needed. In fact, multiple extensions could be defined in one module
and still use string names if entry points are defined pointing to
each Extension subclass.

All of the build-in extensions are now registered with names that
mimic their paths (using dot notation). That may change in the future.
It is expected that all extensions will use short names. We may support
both ('extra' & 'markdown.extensions.extra') in the interim.

Also, all extension configs must now be dicts. Support for a list
of tuples has been removed.