Fixes #359. Thanks for the report @gremmie.

Also removed an inacurate statement in the 2.5 release notes.

Both `safe_mode` and `html_replacement_test` keywords are
pending deprecation, as are positional args. Closes #337.

The builtin extensions will no longer get special treatment and have
the path ("markdown.extensions.") appended . The same applies for
"mdx_" extensions. All names extension must provide the full path.
Fixes #336.

Also deprecating support for passing in extension config settings
as part of the string name. The extension_configs keyword should
be used instead. Fixes #335.

Also raising PendingDeprecationWarnings for positional args or the
"config" keyword on the Extension Class. Pass each setting as a
seperate keyword instead.

Docs and tests are updated. Still need to update extension API docs.

This is in anticipation of #335. The reference and extension api docs still
need to be updated, but that will happen with change in the code.

Except were "short names" are explained in the docs, all references
to the buitlin extensions now use `markdown.extensions.*` in
anticipation of #336.