Markdown2

- [pull 243] task list extra visual changes
- [pull 245] Don't let "target-blank-lines" break footnotes
- [pull 247] Translatable footnote titles
- [pull 252] Add pipe escaping in table extension

- [pull 236] Fix for safe_mode links regression
- [pull 235] Fix for overgreedy regex in metadata
- [pull 237] Fix for header-ids extra non-alpha character issue

- [pull 204] toc extra Python 3 error
- [pull 207] Performance improvements
- [pull 210] Leading paragraph with fenced code blocks
- [pull 212] Target blank links extra
- [pull 215] Optional metadata fences
- [pull 218] Github style task list
- [pull 220] Numbering extra
- [pull 224] Metadata in blocks
- [pull 230] safe_mode changes

- [pull 131] Markdown "spoiler" extra
- [pull 170] html-classes support for table tags
- [pull 190] "strike" extra
- [pull 201] Allow empty table cells

- New "tables" extra for table syntax that matches GFM
<https://help.github.com/articles/github-flavored-markdown#tables> and
PHP-Markdown Extra <https://michelf.ca/projects/php-markdown/extra/#table>.
For example:

| Header 1 | *Header* 2 |
| -------- | -------- |
| `Cell 1` | [Cell 2](http://example.com) link |
| Cell 3 | **Cell 4** |

See <https://github.com/trentm/python-markdown2/blob/master/test/tm-cases/tables.text>
for examples and edge cases.

If you have documents using the 'wiki-tables' syntax and want to convert to the
'tables' syntax, there is a script to help with that here:
<https://github.com/trentm/python-markdown2/blob/master/tools/wiki-tables-to-tables.py>

- [issue 165] Fix an edge case in list parsing.