Added --subsrequired option

If --subsrequired is specified, programme will not be downloaded if
subtitles are not available.
Changed wording of --future description

Inserted "Additionally" to indicate future schedule is searched in
addition to current programmes.
Revert "Handle multiline subtitles"

This reverts commit f495e0d6df5a829c8b1aaa8c3b7e979066e0f1bc.
Subtitle fixes and new formats

Added support for multiline subtitles. (h/t J Larmour)
Fixed small timing error. (h/t R Dixon)
Added --subsfmt to select subtitles format

Possible values for --subsfmt:

default (or --subsfmt omitted):  Line break followed by leading
hyphen demarcates change in speaker.  All other line breaks removed,
so lines must be wrapped by media player.  Text rendered in default
subtitle colour for media player.

compact: Leading hyphen (without line break) demarcates change in
speaker.  All line breaks retained.  Text rendered in default subtitle
colour for media player.