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.

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

The `--extension_configs` option must point to a YAML or JSON file.
The contents of the file must parse to a Python Dict which will be
passed to the `extension_configs` keyword of the `markdown.Markdown`
class.

Also added tests for all of the CLI option parsing options and updated
documentation.

The documentation uses features of Python-Markdown that are not supported on
GitHub and it's better to get a source view of the docs anyway. For example,
that way comments and bug reports can reference a specific line of a file.

Of course, it makes sense for Github to render the README, so that is left
with the `.md` file extension.