Add `--format` to `post`, `comment` and `update`

Acceptable formats are "markdown" ("md" for short), "moinmoin" ("mm" for
short) and "plaintext".
Qualify imported variable in handlers.py
Compare strings properly
Add `default_format` user setting
Make `post` and `update` work without `-f`
Create `SimpleSetting` subclass for `format` setting
Merge pull request #80 from Minoru/disableable-markdown

Changeable formatting