allow for -vf within mplayer_additional_options

[rogerdpack]

Hello. It appears that if mplayer_additional_options contains "-vf scale" or what not, the whole thing is ignored [?]
It might be convenient to allow for such parameters.
Of great convenience, even, would be to have a command-line parameter --additional-mplayer-settings and/or --use-mplayer=/path/to/mplayer.exe

Thank you, of course, I like smplayer a lot!
-roger-