Add custom coefficients for cY, cCb, and cCr

[BoatsMcGee]

The coefficients for the luma, Cb, and Cr planes are hard-coded to 0. This PR allows the user to specify their own coefficients to tweak the characteristics of the Film Grain table.

I have no understanding of the Film Grain specification so this is just an incremental improvement over defaulting to 0, allowing more variation in grain tables without having to extract the generated table, add the values manually, then apply them. Instead, I produce the values by using the diff command, parse the table, then call the generate command with the --cy "VALUES" --ccb "VALUES" --ccy "VALUES" arguments.

Improvements

Initially, I tried to simply pass an existing film grain table file location to an --existing argument for the generate command to parse and replace the values but the parsing wasn't working as expected (no output or error). If the parsing issue could be resolved that would make the generate command cleaner and simplify usage.

Ideally, a set of inputs could be used to generate values besides defaulting to 0. Without an understanding of the specification, simply copying known good values is the best I can do.

Thanks,
- Boats M.