Fixes #712.

The smart_strong extension has been removed and its behavior is now the
default (smart em and smart strong are the default). The legacy_em
extension restores legacy behavior (no smart em or smart strong).

This completes the removal of keywords.  All parser behavior is now
modified by extensions, not by keywords on the Markdown class.

Serializer should only escape & in attributes if not part of & 

Better regex avoid Unicode and `_` in amp detection.

In general, we don't want to escape already escaped content, but with code content, we want literal representations of escaped content, so have code content explicitly escape its content before placing in AtomicStrings.

Closes #669.

Fixes #435.

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.

If you have existing documents that use the legacy attributes format,
then you should enable the legacy_attrs extension for those documents.
Everyone is encouraged to use the attr_list extension going forward.
Closes #643. Work adapted from 0005d7a of the md3 branch.

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.