Changeable formatting

Adds `--format`/`-f` options to `post`, `comment` and `update`. Accepted formats are "markdown" (or "md" for short), "moinmoin" (aka "mm") and "plaintext". There is a setting called `default_format` (set using `-f` parameter of `set`) that specifies which formatting is used by default. If none is specified, and no parameters are passed, "plaintext" is used.