Add in create:channel command

Incidentally this pull request also brings the develop branch up to date with the current master, which I think you would want?

New channel command mimics the admin_content.php 'channel_update' method. It accepts a required argument for the channel short name and not the channel title. This is so it can accept additional arguments. The channel title is then created automatically by replacing underscores with spaces and capitalizing the first word. 

Field group argument is optional and only accepts an ID number. Think some improvements could be made down the line to give options for selecting field groups instead of just passing in a field id. It seems the ````ee()->functions->fetch_site_index()```` which is used for the channel_url returns the site index as http://localhost and not whatever the site url actually is.