Ctrl-X: Cut line on no selection

As discussed is issue #2017 and at trello https://trello.com/c/XVyzky6d

This enables the use of Ctrl-X to cut lines when no text is selected, just like in Sublime, IntelliJ and other modern editors.

Since cutting is done by the shell, this fix simply selects the whole line of each cursor that doesn't have a selection, before letting the shell do it's thing. This results in the desired behaviour.

So to sum up: 
* If you have a cursor at a line (with no selection), hitting Ctrl-X will cut the whole line. 
* If you have a text selection, hitting Ctrl-X will work normal as before. 
* Any combinations with multiple cursors is supported.