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

docs/userguide

@@ -1191,6 +1191,31 @@ bar {
 }
 ------------------------
 
+=== Strip workspace numbers
+
+Specifies whether workspace numbers should be displayed within the workspace
+buttons. This is useful if you want to have a named workspace that stays in
+order on the bar according to its number without displaying the number prefix.
+
+When +strip_workspace_numbers+ is set to +yes+, any workspace that has a name of
+the form "[n]:[NAME]" will display only the name. You could use this, for
+instance, to display Roman numerals rather than digits by naming your
+workspaces to "1:I", "2:II", "3:III", "4:IV", ...
+
+The default is to display the full name within the workspace button.
+
+*Syntax*:
+----------------------------------
+strip_workspace_numbers <yes|no>
+----------------------------------
+
+*Example*:
+----------------------------
+bar {
+    strip_workspace_numbers yes
+}
+----------------------------
+
 === Binding Mode indicator
 
 Specifies whether the current binding mode indicator should be shown or not.