epub: Allow manual chapter splits

Default format entry
`s` or `m:s` or `h:m:s`
e.g. `18:21` will split 18 minutes and 21 seconds in. I don't think any value needs to be upper-bound e.g. `120` is equivalent to `2:0' and will split at 2 minutes in.
All values should be non-negative integers, except seconds which may also includen a decimal part (see below)

It would be useful to specify multiple splits by separating with whitespace or commas. Blank or invalid entries would be ignored. "e.g. ,,"

A frame that falls exactly on the specified time belongs _after_ the split, not before.

How should we handle sub-second splits? There are two options to investigate - fraction of a second or frame number e.g.
`18:21.95` or `18:21#20`
However frames don't necessarily exactly divide into seconds, so I'm not sure that the latter one is valid or useful.