Allows download to proceed with default version even if bogus AD/SL
version appears in programme metadata. Many occasionally be incorrect.

BBC has begun redirecting more playlists. Redirect tracking is necessary
to construct correct media playlists

Streams no longer exist and BBC blocks playlist URLs with 403 errors.
--hls-lq-audio is now ignored.

Running on perl 5.16 I get the following error:

Error in option spec: "cuesheetonly||cuesheet-only!"

This appears to be due to use of | rather than ||, so this change
adjusts the code to avoid the error.

Adapted for BBC changes

also add "legal" to hard-coded default version list

Apply --pid-recursive-type with --subtitles-only, --metadata-only, etc.

Use --cuesheet to create cue sheet (.cue file) for downloaded programme.
Use --cuesheet-only to create cue sheet without programme download.
Tracklist and cuesheet now built solely from JSON segments file.

If used, --pid-recursive-type limits recursive download to programmes
of specified type (radio or tv. Requires --pid-recursive.

get_iplayer can now check GitHub for new releases. If a new release is
found, a message is printed. get_iplayer is NOT automatically updated.
Use new --release-check option on the command line to force check for
new release. If --release-check is saved in preferences, GitHub will
be automatically checked weekly. The last release check time is taken
from mod time of "release_check" file in profile dir. Requires 'version'
Perl module, which entered core at v5.9.0.

Use --metadata=json to produce metadata file in JSON format.

Ensure leading hyphens used to denote change of speaker in embedded
subtitles since they will be rendered in single colour.

Related versions placed together in list, and podcast versions consistently
given lowest priority by default.