Initial YUV4MPEG2 output support.
More flexible framerate.

YUV4MPEG2 output framerate can now be set with -f option.
-f option now supports fractional framerates as a ratio A:B, a fraction
C/D or decimal E.F
Use same variable to signal y4m for in- and output
Fix default y4m output filename extension.
Various y4m fixes.

* Seek to StartFrame if specified.

* Stop encoding when End Of File is reached.

* Use more standard single dash for stdin.
Merge pull request #2 from Vproject/y4moutput

Y4moutput