Add a webm profile

Add the ability to generate WebM files using ffmpeg. This involved
refactoring the code a little but should ease the addition of future
profiles.

You now need to pass -P profile, where profile is currently either
theora or webm.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
Add a -h option

This shows the basic program usage.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>