mediawiki format: literal pipes in cells/headers silently split columns #499

What breaks

tablefmt="mediawiki" does not escape | in cell or header content, so any literal pipe silently splits the cell into extra columns on a real MediaWiki wiki:

>>> import tabulate
>>> print(tabulate.tabulate([["x | y"], ["a || b"]], headers=["h | 1"], tablefmt="mediawiki"))
{| class="wikitable" style="text-align: left;"
|+ <!-- caption -->
|-
! h | 1
|-
| x | y
|-
| a || b
|}

Rendered on a wiki, every pipe becomes a column separator: the header row gains a spurious empty column, x | y becomes two cells, and a || b becomes three — the data is silently corrupted.

This is the same class as the escaping the github/pipe formats already do for their separator (x | y is rendered as x \| y there), and the same class prettytable fixed for its MediaWiki emitter (prettytable #544): a literal pipe inside a MediaWiki table cell must be written |.

The header row has the same problem with !: a header cell containing !! splits into extra header columns.

Fix direction

Escape | as | in cell and header values in _mediawiki_row_with_attrs (and ! as ! in header rows), the way _html_row_with_attrs HTML-escapes values for the html/unsafe_html formats. Newlines inside cells are a separate, already-tracked multiline-support TODO, so they are out of scope here.