Provide additional useful Ex commands

ElScreen provides additional functionality that is usefully exposed as Ex commands.  I've provided `:tabclone` to create a new tab with the same window splits as the cloned tab, `:tabdisplay` to toggle the display of the tabs, `:tabgoto` to switch between tabs by number (like 2gt), `:tabonly` to close all tabs but the current one, `:tabrename` to name or rename a tab, `:tabselect` to choose a tab to switch to from a menu list, and `:tabwith` to switch to the first tab currently showing the named buffer.