Refactor parsing of CLI args so that we can modify them in the base class

Implement tag and skip_tag handling in the CLI() class.  Change tag and
skip_tag command line options to be accepted multiple times on the CLI
and add them together rather than overwrite.

* Make it configurable whether to merge or overwrite multiple --tags arguments
* Make the base CLI class an abstractbaseclass so we can implement
  functionality in parse() but still make subclasses implement it.
* Deprecate the overwrite feature of --tags with a message that the
  default will change in 2.4 and go away in 2.5.

* Add documentation for merge_multiple_cli_flags
* Fix galaxy search so its tags argument does not conflict with generic tags
* Unit tests and more integration tests for tags

lib/ansible/cli/doc.py

@@ -60,7 +60,8 @@ class DocCLI(CLI):
         self.parser.add_option("-s", "--snippet", action="store_true", default=False, dest='show_snippet',
                 help='Show playbook snippet for specified module(s)')
 
-        self.options, self.args = self.parser.parse_args(self.args[1:])
+        super(DocCLI, self).parse()
+
         display.verbosity = self.options.verbosity
 
     def run(self):