Updated tests for Shot().append_vf() and Shot().append_af().

Test driven development. TODO:
Shot().add_filter() is replaced with Shot().append_vf();
Shot().append_af() is added;
Keyword argument "filters" in Shot constructor is removed;
Keyword arguments "vf" and "af" are added;
YAML interpreter replaces "filters" with "vf" in output.
Added support for audio filters.