Handle memberlist updates in channelsinfo message
Handle memberlist updates in channelsinfo message

In large Slacks, channel membership isn't included in the initial channel
info, but in channelsinfo messages sent after connection. This fixes an
issue where most users would not ever show up in the nicklist when
connecting to such slacks.

trygveaa/wee-slack,wee-slack/wee-slack,rawdigits/wee-slack