Subteams are stripped from the message

[andrecloutier-pd]

Given a message from Slack like: FYI <!subteam^S0TR1SJKX|@imps> test
Lita will receive the message as: FYI test
I would expect it to receive: FYI @imps test or preferably FYI <@S0TR1SJKX> test per the docs.

The issue seems to stem from https://github.com/litaio/lita-slack/blob/master/lib/lita/adapters/slack/message_handler.rb#L55 not handling subteam properly.