[Feature] Add a second karaoke audio track to video

[suntong]

Better be able to add a karaoke audio as the second audio track to the MTV video.

[suntong]

karaoke cli options:

ffcvt/test/ffcvt_test.txt

Line 38 in b1ec953

-K,karaoke Add a karaoke audio track to .mp4 MTV (FFCVT_K,KARAOKE)

usage examples:

ffcvt/test/test-all.sh

Lines 28 to 29 in b1ec953

	$FFCVT -n -f test1.avi -t '' -K >> /tmp/ffcvt_test.txt 2>&1
	$FFCVT -n -f test1.avi -t '' -karaoke -ext _k.avi >> /tmp/ffcvt_test.txt 2>&1

will output to test1_karaoke.mp4 and test1_k.avi respectively.