moinmoin and textile formats: literal pipes in cells silently split columns #501

What breaks

tablefmt="moinmoin" and tablefmt="textile" write cell and header values raw, so a literal pipe in the content silently splits the cell — the same class as the mediawiki problem in #499, but with these two formats' own separators:

>>> import tabulate
>>> print(tabulate.tabulate([["x | y", "a || b"]], tablefmt="moinmoin"))
||<style="text-align: left;">  x | y    ||<style="text-align: left;">  a || b  ||

|| is the cell separator in MoinMoin tables, so a || b becomes three cells and the style attribute of the second one ends up on a phantom cell. In textile, | is the separator, so x | y splits in two. Rendered on a real MoinMoin wiki or through a textile converter, the data is silently corrupted.

Fix direction

Escape | as | in the cell and header values of both formats — MoinMoin and Textile both pass numeric character references through and render them as the literal character — the same approach the mediawiki fix (#500) uses.