dh addon interface

dh addon interface

* dh: Add an interface that third-party packages providing debhelper commands
  can use to insert them into a command sequence.
  (See dh(1), "SEQUENCE ADDONS".)
* dh: --with=foo can be used to include such third-party commands.
  So, for example, --with=cli could add the dh_cli* commands from
  cli-common.
* Moved python-support special case out of dh and into a python-support
  sequence addon. --with=python-support is enabled by default to avoid
  breaking backwards compatability.