Allow to filter tweets from @MAINACCOUNT of a chan

!filter @account now filters only tweets sent by the user @account not those sent to him (use filter account for that)
Now also works with a chan's twitter account (although, if unset, messages mentionning the account will always get through even if they match any other filter set to avoid missing these)

Considering RTs can already be filtered separately for the account's chan and for all tweets, I believe this closes #10