Add simple 'extends' option to configuration

Allow a Docker Container configuration to extend another.
All top level config elements from the extended container
will be copied over to the new configuration unless the new
configuration has defined them.

    tomcat:
      image: x
      exposedPorts: [8080/tcp]
    tomcat2:
      extends: tomcat
      image: y

tomcat2 will have the same exposedPorts as tomcat, but a different image.

fixes #32
Merge pull request #34 from aslakknutsen/32-extend-config

Add simple 'extends' option to configuration