options: support chapters for --start and --end

The --start and --end switch now accept a chapter number. The chapter
number is prefixed with '#', e.g. "--start=#2" jumps to chapter 2.

The chapter support might be able to replace --chapter completely, but
for now I am not sure how well this works out with e.g. DVDs and BDs,
and a separate --chapter option is useful interface-wise.

(This was supposed to be added in 51503a, but apparently the fixup
commit adding it was lost in a rebase. This might also be the reason
for the mess-up fixed in 394285.)