New `toc_tokens` attribute on Markdown class.
Contains the raw tokens used to build the Table of Contents. Users can 
use this to build their own custom Table of Contents rather than needing 
to parse the HTML available on the `toc` attribute of the Markdown 
class.

And other docs cleanup.

Additional CSS classes names can be appended to the admonition name
using spaces as separators.

The following markdown:

    !!! note floatright
        This is a floating note.

Generates the following HTML code:

    <div class="admonition note floatright">
        <p class="admonition-title">Note</p>
        <p>This is a floating note.</p>
    </div>

Add new InlineProcessor class that handles inline processing much better and allows for more flexibility. This adds new InlineProcessors that no longer utilize unnecessary pretext and posttext captures. New class can accept the buffer that is being worked on and manually process the text without regex and return new replacement bounds. This helps us to handle links in a better way and handle nested brackets and logic that is too much for regular expression. The refactor also allows image links to have links/paths with spaces like links. Ref #551, #613, #590, #161.

The page was removed along with the extension in 1127013.

Also ensure any future MkDocs warnings cause the build to fail in the tests
so we don't introduce similar errors in the future.

Deprecated naming support is removed:

* Removed special treatment for modules in `markdown.extensions`
* Removed support for `mdx_` prefixes.

Support for Entry Point names added:

Support for "short names" are now implemented with entry points. 
Therefore all the users who call extension names as `toc` will not 
get errors as the builtin extensions all have entry points defined 
which match the old "short names" for modules in 
`markdown.extensions`. The benefit is that any extension can offer
the same support without requiring the user to manually copy a file 
to that location on the file system (way to many extension authors 
have included such instructions in their installation documentation).

The one odd thing about this is that we have been issuing a 
DeprecationWarning for short names and now they are fully supported 
again. But I think it's the right thing to do.

Support for using dot notation is not removed. After all, it was never 
deprecated. And we shouldn't "force" entry points. There are plenty of 
reasons why users may not want that and not all of them can be 
resolved by using class instances instead.

All of the following ways to load an extension are valid:

    # Class instance
    from markdown.extensions.toc import TocExtension
    markdown.markdown(src, extensions=[TocExtension()]

    # Entry point name
    markdown.markdown(src, extensions=['toc'])

    # Dot notation with class
    markdown.markdown(src, extensions=['markdown.extensions.toc:TocExtension'])

    # Dot notation without class
    markdown.markdown(src, extensions=['markdown.extensions.toc'])