Rewrite command parsing / interface

[tribut]

Just accidently typed "/otr smp respond some secret" (unquoted secret) and received a very unhelpful backtrace:

python: stdout/stderr: Traceback (most recent call last):
python: stdout/stderr:   File "/home/weechat/.weechat/python/autoload/otr.py", line 1344, in command_cb
python: stdout/stderr:     if secret:
python: stdout/stderr: UnboundLocalError: local variable 'secret' referenced before assignment

The simple fix would be to suggest to use quotes in the hint («Respond with: /otr smp respond "<answer>"»?). But we should probably look at all the commands again and see if we can change the interface to avoid those kind of ambiguities. This might also a good opportunity to look at the command parsing code in general. It is still very hard to read and audit.

[mmb]

Good find. I wrote some test on a branch to catch these types of issues:

https://github.com/mmb/weechat-otr/tree/arg_parse_fuzz

We can use it to test the fix and any refactors.