awful.rules: Add the 'tags' property

Due to recent changes, it was no longer possible to disable the
default tag selection handler. This commit extend the already
existing request::tag mechanism to let handlers select the tags.

lib/awful/rules.lua

@@ -16,6 +16,7 @@ local type = type
 local ipairs = ipairs
 local pairs = pairs
 local atag = require("awful.tag")
+local util = require("awful.util")
 local a_place = require("awful.placement")
 
 local rules = {}
@@ -356,6 +357,11 @@ function rules.extra_properties.placement(c, value)
     end
 end
 
+function rules.extra_properties.tags(c, value)
+    local current = c:tags()
+    c:tags(util.table.merge(current, value))
+end
+
 --- Apply properties and callbacks to a client.
 -- @client c The client.
 -- @tab props Properties to apply.