Escape pipe characters in table cells

[gaoflow]

Cell content containing a literal | is emitted unescaped, so the pipe is parsed as a column delimiter and the table gains spurious columns:

>>> md('<table><tr><th>A</th><th>B</th></tr><tr><td>1|2</td><td>3</td></tr></table>')
'\n\n| A | B |\n| --- | --- |\n| 1|2 | 3 |\n\n'

(three data cells under a two-column header, and everything after the stray pipe lands in the wrong column)

This escapes | as \| in convert_td/convert_th, skipping pipes that escape_misc=True has already escaped. It's done in the cell converters rather than in escape() so pipes inside code spans are covered too: GFM splits rows into cells before inline parsing, so `1|2` also breaks the row and has to be written as `1\|2` (checked against cmark-gfm via pandoc, which escapes cell pipes the same way).