avconv_opt: Allow printing private options

avconv_opt: Allow printing private options

Add an allowed parameter to -h and --help avconv option to print private
options from a codec, format, or filter, named with the provided input
value.

In case multiple classes are found (eg. mov demuxer and mov muxer, or
h264 decoder and h264 demuxer) print all options from all classes.
It is possible to select the type of class to print by adding it
before the name (eg. demuxer:mov and muxer:mov, or decoder:h264and
demuxer:h264).

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>