avocado.multiplexer: Support for multiple yaml files V0

__Beta__ version of the multiple yaml files support. I have some difficulties with the new HTML plugin (utf-8) characters which I need to fix before fixing and updating `selftests`. Please take a look at the concept and let me know, what you think of it.

https://trello.com/c/VFlrQk9R/119-multiplexer-support-for-multiple-files