Add a config to set the footnote separator. Also remove rel/rev as they aren't really compatible with HTML5 and we already have classes set for refs and backrefs.

Fixes 723.

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.

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.

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.

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.

Preserve the line which a reference was on to prevent raw HTML indexing issue. Fixes #584.

Prevent raw HTML parsing issue in abbr and footnotes

Peserve abbreviation line when stripping and preserve a line for each footnote block.  Footnotes should also accumulate the extraneous padding.

Test extra lines at the end of references

Strip the gathered extraneous whitespace

When processing footnotes, we don't actually care to process the extra whitespace at the end of a footnote, but we want it to calculate lines to preserve.

Fixes #618.

Fixes #610.