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

i3bar/include/config.h

@@ -27,6 +27,7 @@ typedef struct config_t {
     struct xcb_color_strings_t colors;
     bool         disable_binding_mode_indicator;
     bool         disable_ws;
+    bool         strip_ws_numbers;
     char         *bar_id;
     char         *command;
     char         *fontname;