Switch contexts

[marek-saji]

Allow bot to switch contexts — i.e. bot may ask for something and wait for an answer.

• There may be two kinds of contexts:
  1. strict — limit only same plugin that started context to leave it
  2. loose — give the plugin priority to handle responses, but if not handled, other plugins may handle it, which leaves context
• Separate contexts for channels/users. But plugin may also want to change the context for whole channel (should be possible only for loose).
• Auto leave-contexts after some time(?)