Provide a .botnet command to add a botnet ban

Summary:
.botnet <nick> allows to directly ban a botnet
automated client, without waiting SendQ or excess
flood quit message.

Test Plan:
Tested on a botnet candidate and on myself,
got a regular ban for the first, an error
message in the second case, as expected.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D961