: search(keys, charset = nil)
: uid_search(keys, charset = nil)
      Sends a SEARCH command to search the mailbox for messages that
      match the given searching criteria, and returns message sequence
      numbers (search) or unique identifiers (uid_search).

      ex).
        p imap.search(["SUBJECT", "hello"])
        #=> [1, 6, 7, 8]
        p imap.search('SUBJECT "hello"')
        #=> [1, 6, 7, 8]

