Add ability to convert segment of a cast

[sandorex]

I've based this on #110, but its trivial to port it to main

This is another feature on my wishlist, basically select start and end frame to output as gif (or frames)

[ku1ik]

Hey @sandorex,

Thanks for the PR.

I believe it's best to decouple frame selection from output format, and I've been thinking about how to tackle this for a while. I've been looking for a good generic and flexible solution for frame selection (#111 also attempted this), and I believe I found something that should work well: #116 - let me know what you think.

Alternative output formats (include individual frames as PNG) is definitely something I want to support, and that's something like to include in the following release (while the next one - 1.9 - will be about frame selection only I think). I have some ideas for this, will share soon.