Feat Req: Add flag to convert output to specified length

[bear-jordan]

Overview
Hi, great tool — I mostly use vhs to demonstrate code / successful builds on pull requests. I often convert my videos from their full runtime down to a few seconds so the reviewer can see the behavior.

Suggested Solution
It would be great if the user could pass in the desired output length as a parameter when calling vhs.

# Add -s flag for seconds or shrink?

vhs <file> -s <desired time in seconds>

Alternatives
Right now, I just use this script to perform the action (link to gist)

Context
Unfortunately, I don't have time to implement it, but I wanted to see if anyone else was interested