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_directives.h

@@ -81,4 +81,5 @@ CFGFUN(bar_color_single, const char *colorclass, const char *color);
 CFGFUN(bar_status_command, const char *command);
 CFGFUN(bar_binding_mode_indicator, const char *value);
 CFGFUN(bar_workspace_buttons, const char *value);
+CFGFUN(bar_strip_workspace_numbers, const char *value);
 CFGFUN(bar_finish);