Adds button for joining a channel without writing /join

Fixes issue #123 and adds a button under the channel list for easily inputing a
channel name and pressing enter. This should make it more easy for new users to
IRC, joining channels. Someone with a better feel for UI may want to look at
how to make this prettier and even simpler.
Merge pull request #300 from hermansc/new-channel

Adds button for joining a channel without writing /join