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

src/ipc.c

@@ -514,6 +514,9 @@ static void dump_bar_config(yajl_gen gen, Barconfig *config) {
     ystr("workspace_buttons");
     y(bool, !config->hide_workspace_buttons);
 
+    ystr("strip_workspace_numbers");
+    y(bool, config->strip_workspace_numbers);
+
     ystr("binding_mode_indicator");
     y(bool, !config->hide_binding_mode_indicator);