Fixes #601. Merged in 6f87b32 from the md3 branch and did a lot of cleanup.

Changes include:

* Removed old docs build tool, templates, etc.

* Added MkDocs config file, etc.

* filename.txt => filename.md

* pythonhost.org/Markdown => Python-Markdown.github.io

* Markdown lint and other cleanup.

* Automate pages deployment in makefile with `mkdocs gh-deploy`

    Assumes a git remote is set up named "pages". Do

        git remote add pages https://github.com/Python-Markdown/Python-Markdown.github.io.git

    ... before running `make deploy` the first time.

Ancestry exclusion for inline patterns.

Adds the ability for an inline pattern to define a list of ancestor tag names that should be avoided. If a pattern would create a descendant of one of the listed tag names, the pattern will not match. Fixes #596.

Also consume to the end of the first line of any admonition. Everything after
the title is discarded. However, the only thing that will match should be
whitespace so it should be a non-issue.
Fixes #550.

Single column tables are valid tables, so add back in the accidentally
removed functionality of allowing single column tables, but with one
exception -- table bodies should not render empty (an empty 
`<tbody>` is invalid HTML. Fixes #539.

If both open and close was not found in first block, additional blocks
were evaluated without context of previous blocks.  The algorithm needs
to evaluate a buffer with the left bracket present.  So feed in all
items and get the right bracket, then adjust the data_index to be
relative to the last block. Fixes #452.

Fixes #412 and #493.  First we parse footnote content as
its own document avoid quirks with using li as a parent. Second, we
surround placeholders with STX and ETX to prevent them from interfering
with inline parsing; this is also consistent with how placeholders are
used everywhere else in Python Markdown.

Track when we find duplicate footnote references and create unique ids
for them.  Then add an additional tree-processor after inline to go
back and update the footnotes with additional back references that link
to the duplicate footnote references. Fixes #468.

HRProcessor tried to access a member variable after recursively calling
itself.  In certain situations HRProcessor will try to access its
member variable containing its match, but it will not be the same match
that call in the stack expected.  This is easily fixed by storing the
match locally *before* doing any recursive work.

This aims to escape code in a more expected fashion.   This handles
when backticks are escaped and when the escapes before backticks are
escaped.