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
1efe782b46