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.

All file-based tests are now defined as unittest test cases via a
metaclass which walks a directory and builds a unittest for each pair
of test files.

To run the tests just run `python -m unittest discover tests`. Or use
tox as the tox config has been updated to run the new tests and all nose
specific code has been removed.

The test generator tools have been removed as well. If any changes or
additions need to be made to tests, they should be implemented using
the new framework rather than with the file-based tests. Eventually,
only the PHP and pl tests should remain as file-based tests.

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.

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.

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.

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

Tables now handle escaped pipes when testing, in table borders, and in
the inline content.  To achieve properly, a bug had to be fixed related
to appending escaped chars to the Markdown class.  Now appended chars
only appear in the current instance.  Lastly the first backtick in a
table can be escaped rounding out the last corner case.

At some point the logic of counting backticks and determining if they are odd or even was used to parse a row's text into cells. Unfortunately this approach broke expected code parsing logic in a table. We essentially traded one bug for another. This fixes table backtick handling and restores sane backtick logic while preserving existing fixes. (issue #449)