stats module

Prints dts, segment number, length, difference to previous dts and md5
of block.

with prefix-option you can give prefix to print in stats-line, so you can
add stats module multiple times with different prefixes. Output to file
is given with output parameter.
transcode: don't try to setup transcode chain if there isn't audio codec defined
transcode: only try to create transcode chain if video codec is defined
transcode: check codec aliasing for SPU also
fdkaac: prefer fdkaac over avcodec for mp4a encoding