RFC: multiplexer: Construct the tree directly from yaml file [v0]

Hi guys,

this __PoC__ should demonstrate how to create tree directly from yaml file by custom `yaml.constructor`. It's incompatible with the https://github.com/avocado-framework/avocado/pull/271 but the combination can be done while merging the trees. In this version it only supports tree creation and the only tested command was:

`avocado/avocado multiplex -c ../examples/mux-environment.yaml`

Please have a look and let me know, if you like it. It should be easy to combine with custom types which can separated the `mux:` or even `filter-out` (I personally don't like that `filter-out` is part of the environment. IMO it would be better to specify `yaml` tag for it and store them in tree structure. But more about this later...)

Kind regards,
Lukáš

PS: Sorry for the style cleanups, it's only that my Eclipse doesn't like long lines. Yes, it's only an excuse, because actually I don't like them and I set the Eclipse to fix them automatically...