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

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

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)