Add 'suggested_channels' option & focus on first input on connect dialog

I'm not sure about the name "suggested_channels" for the new option.  I considered "preset_channels" to be consistent with "preset_server", but unlike "preset_server" you can join other channels not listed in "suggested_channels".

Also, I'm not really experienced in web development (or pull requests!) so if anything is bad, let me know.  Ideally I want to conform to whatever style you had in mind for relay.js and learn a little about web stuff at the same time.