Added check for empty subtitles
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 unspecified):  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.

break: Like default except that line breaks in subtitles file
are retained.  This may lead to some awkward formatting due to the
extra line breaks inserted for changes in speaker.

basic: No demarcation of change in speaker.  All line breaks retained.
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.

colour (or color): Change in text colour demarcates change in
speaker.  All line breaks retained.  Text rendered in colours defined
in subtitles file.  Any text for which a colour cannot be ascertained
is rendered in default subtitle colour for media player.