[Request] Break source into segments for recoverable conversions

@cdgriffith,

Great software! I wrote my own h.265 UHD conversion software before I ever heard of this, but this is much better than mine!

However, I tend to only convert overnight, so I like to have spots in the conversion that are easy to break and recover from. Is there any feature like this in this project?

FYI, To do this in my implementation, I first segment the source file into 3 minute video-only chunks (and a list) with the following parameters in the output string:

`-f segment -segment_time_delta 5 -segment_time 180 -segment_list `

Then, I loop through the resulting files, converting them one at a time and finally concatenating all of the segments back together using the segment list.

I can help implementing this here if it is useful, but I am not sure exactly how to fit it into your UI, especially since this should be able to work in between sessions. This also might help with #227