Add libvpx presets.

Based on a patch by James Zern jzern AT google DOT com

ffpresets/libvpx-1080p.avpreset

+g=120
+lag-in-frames=16
+deadline=good
+cpu-used=0
+profile=1
+qmax=51
+qmin=11
+slices=4
+b=2M
+
+#ignored unless using -pass 2
+maxrate=24M
+minrate=100k
+auto-alt-ref=1
+arnr-maxframes=7
+arnr-strength=5
+arnr-type=centered

ffpresets/libvpx-1080p50_60.avpreset

+g=120
+lag-in-frames=25
+deafline=good
+cpu-used=0
+profile=1
+qmax=51
+qmin=11
+slices=4
+b=2M
+
+#ignored unless using -pass 2
+maxrate=24M
+minrate=100k
+auto-alt-ref=1
+arnr-maxframes=7
+arnr-strength=5
+arnr-type=centered

ffpresets/libvpx-360p.avpreset

+g=120
+lag-in-frames=16
+deadline=good
+cpu-used=0
+profile=0
+qmax=63
+qmin=0
+b=768k
+
+#ignored unless using -pass 2
+maxrate=1.5M
+minrate=40k
+auto-alt-ref=1
+arnr-maxframes=7
+arnr-strength=5
+arnr-type=centered

ffpresets/libvpx-720p.avpreset

+g=120
+lag-in-frames=16
+deadline=good
+cpu-used=0
+profile=0
+qmax=51
+qmin=11
+slices=4
+b=2M
+
+#ignored unless using -pass 2
+maxrate=24M
+minrate=100k
+auto-alt-ref=1
+arnr-maxframes=7
+arnr-strength=5
+arnr-type=centered

ffpresets/libvpx-720p50_60.avpreset

+g=120
+lag-in-frames=25
+deadline=good
+cpu-used=0
+profile=0
+qmax=51
+qmin=11
+slices=4
+b=2M
+
+#ignored unless using -pass 2
+maxrate=24M
+minrate=100k
+auto-alt-ref=1
+arnr-maxframes=7
+arnr-strength=5
+arnr-type=centered