Support ~~~ fences in the fenced-code-blocks extra - #776

The fenced-code-blocks extra only matches backtick fences, so a block fenced with ~~~ (CommonMark and GFM allow both, and Pandoc writes tildes) is not treated as code:

s = "Run:\n\n~~~\nls ~/x\n~~~\n"
markdown2.markdown(s, extras=["fenced-code-blocks"])
# '<p>Run:</p>\n\n<p>~~~\nls ~/x\n~~~</p>\n'
markdown2.markdown(s, extras=["fenced-code-blocks", "strike"])
# '<p>Run:</p>\n\n<p><s>~\nls ~/x\n~</s></p>\n'

The code ends up in a paragraph, and with strike the whole block is struck through.

The opening fence now accepts three or more ~ as well as backticks, the closing fence matches the same opener, and test() also checks for ~~~. Backticks inside a tilde block stay as code.

Added test/tm-cases/fenced_code_blocks_tilde, which fails before the change. The rest of python test.py gives the same results before and after (the existing knownfailure cases).