Add keepFrames argument to cli.js

[varunarora]

The arg doesn't work, even though it is accounted for in index.js

[tungs]

I think the argument was left undocumented (and not used by cli.js) because at the time, I didn't know exactly how I wanted it to work. Thinking about it now, I think the user can provide an optional path or maybe a pattern, otherwise a directory name will be generated. I'll think about it today and write an implementation.

[varunarora]

That could work, Steve! But really, you may not need to overthink it that much. I was thinking of a debug flag, but it's loaded - so definitely not.

[tungs]

I added basic support for this in d01aa4db8c6044213d9cec553784a2c99aecfb76, as a boolean flag, but I should revisit this at some point for optional arguments.

[tungs]

Some considerations, for future reference, about the optional argument:

• It should probably have support for an output directory and an output pattern
• If there already exists a directory/frames should they be overwritten or renamed? The name implies that there is a priority in saving them, so they probably should not be automatically overwritten.