avocado.plugins: Add multiplexer plugin

Add a multiplexer parser, so people can try out their
multiplex files. Here is an example of usage

    $ scripts/avocado multiplex tests/sleeptest/sleeptest.mplx
    Dictionaries generated:
        dict 1:    sleeptest.short
        dict 2:    sleeptest.medium
        dict 3:    sleeptest.long

You can display dict contents by providing the flag --contents
(short -c).

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
.travis.yml: Add multiplex plugin functional testing

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>