Add `weechat-bol'.

Make C-a skip the prompt.  Basically copied from `comint-bol'.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Update `weechat-switch-buffer's docstring.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>