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

parser-specs/config.spec

@@ -358,6 +358,7 @@ state BAR:
   'font'                   -> BAR_FONT
   'binding_mode_indicator' -> BAR_BINDING_MODE_INDICATOR
   'workspace_buttons'      -> BAR_WORKSPACE_BUTTONS
+  'strip_workspace_numbers' -> BAR_STRIP_WORKSPACE_NUMBERS
   'verbose'                -> BAR_VERBOSE
   'colors'                 -> BAR_COLORS_BRACE
   '}'
@@ -420,6 +421,10 @@ state BAR_WORKSPACE_BUTTONS:
   value = word
       -> call cfg_bar_workspace_buttons($value); BAR
 
+state BAR_STRIP_WORKSPACE_NUMBERS:
+  value = word
+      -> call cfg_bar_strip_workspace_numbers($value); BAR
+
 state BAR_VERBOSE:
   value = word
       -> call cfg_bar_verbose($value); BAR