basic twirssi_search implementation

[mmitch]

adds a twirssi_search command to start a one-time search

includes:

• search is queued via background job and a twirssi_updates is started
• queued searches are removed upon the first result (non-existing #hashtags will be searched until they show up or you restart irssi)
• a twitter_search_results setting has been added to set the maximum number of search results
• parallel runs of the background job should be prevented by the $update_is_running flag

  does not include (help/pointers wanted):

• search results are not remembered and not checked against already seen tweets

[mmitch]

Should I use $username = "$user\@$defservice" instead of $username = &normalize_username( $user ) to expand to the current full username?