Mark special treatment of extension names as PendingDeprecation

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.
Updated extension API docs for recent changes.
Some docs cleanup.