Removed double re-muxing of AAC audio (national radio) to M4A

Also added --ffmpeg-<type>-opts to specify custom options for ffmpeg
re-muxing.
Added support for optional prefixes in substitution parameters

Substitution parameters may now contain prefixes before the parameter
name. The prefix will not be printed if the associated token is empty
or undefined.  For example:

--fileprefix="<nameshort><-senum>"

will expand to "The_Last_Station" for the film of the same name since
<senum> is undefined for films.  For Top Gear series 19 episode 2,
where <senum> is defined, it will expand to "Top_Gear-s19e02".
Set TV series/episode numbers to 1 if both undefined and programme not a film

This will ensure that the <senum> substitution parameter will be defined
as "s01e01"for one-off documentaries, but undefined for films.
Added --ffmpeg-<type>-opts to man page
Restrict substitution parameter prefix to non-alphanumeric and underscore
Allows --subtitles to work with --versions=audiodescribed