Refactor casting in Animation.Config module

[vanvoljg]

There are a couple TODO notes for refactoring the casting logic in RGBMatrix.Animation.Config. @amclain, I think you had a suggestion for how we could do this, didn't you?

• TODO: better key casting (lib/rgb_matrix/animation/config.ex#L77)
• TODO: better value casting (lib/rgb_matrix/animation/config.ex#L80)

[amclain]

This is a duplicate of #70. I recommend we close this issue and move the checkboxes to the other one.

@amclain, I think you had a suggestion for how we could do this, didn't you?

I made a comment to log the tech debt in the PR: #56 (comment)

I also jotted down some pseudo-code for cast, which I believe was implemented: #54 (comment)

[vanvoljg]

I thought it was somewhere, but missed it in the list. Thanks.