i3bar: implement custom workspace numbers config

Implement the configuration option within the bar config directive for
custom workspace numbers with the directive `strip_workspace_numbers
yes`.

This directive strips the workspace name of the number prefix and
delimiter. When the workspace name consists only of the number, it will
default to show the number.

For example:

* "2:5" -> "5"
* "4:$" -> "$"
* "8" -> "8"

This allows customization of i3bar for alternate ordering of workspaces
which has a legitimate use for alternate keyboard layouts such as
Dvorak.

fixes #1131

include/config.h

@@ -267,6 +267,10 @@ struct Barconfig {
      * zero. */
     bool hide_workspace_buttons;
 
+    /** Strip workspace numbers? Configuration option is
+     * 'strip_workspace_numbers yes'. */
+    bool strip_workspace_numbers;
+
     /** Hide mode button? Configuration option is 'binding_mode_indicator no'
      * but we invert the bool for the same reason as hide_workspace_buttons.*/
     bool hide_binding_mode_indicator;