MAINTAINERS: add myself for libvpx*

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
libvpxenc: add VP9 options

same as their vpxenc equivalents:
-lossless
-tile-columns
-tile-rows
-frame-parallel

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>