Closes #786.

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.

In the past, most of the config was defined using globals. Today all of 
the config is held on the class instance. Therefore, the `md_globals` 
parameter is no longer necessary.

Previously, instances of the Markdown class were represented as any one 
of 'md', 'md_instance', or 'markdown'. This inconsistency made it 
difficult when developing extensions, or just maintaining the existing 
code. Now, all instances are consistently represented as 'md'.

The old attributes on class instances still exist, but raise a 
DeprecationWarning when accessed. Also on classes where the instance was 
optional, the attribute always exists now and is simply None if no 
instance was provided (previously the attribute wouldn't exist).

All processors and patterns now get "registered" to a Registry.
Each item is given a name (string) and a priority. The name is for
later reference and the priority can be either an integer or float
and is used to sort. Priority is sorted from highest to lowest. A 
Registry instance is a list-like iterable with the items auto-sorted 
by priority. If two items have the same priority, then they are 
listed in the order there were "registered". Registering a new 
item with the same name as an already registered item replaces
the old item with the new item (however, the new item is sorted by
its newly assigned priority). To remove an item, "deregister" it by 
name or index.

A backwards compatible shim is included so that existing simple
extensions should continue to work. DeprecationWarnings will 
be raised for any code which calls the old API.

Fixes #418.

In the past Markdown used to pass extension config settings to the
Extension class via a positional argument named `config`. That was
deprecated in 2.6 in favor of using keyword arguments (`**kwargs`).
Support has been completely dropped. Only keyword arguments are
accepted.