Added deepProp, deepTheme, and deepPalette

[jtmthf]

Re-closes #55. Adds deepProp, deepTheme, and deepPalette.

[diegohaz]

Thank you again @jtmthf. I made a few suggestions (a yarn lint --fix may be needed after applying them).

Also, I was thinking about, instead of separate functions, a single function resolvePaletteProps that would return { key, tone, defaultValue } and then used as:

const { key, tone, defaultValue: finalDefaultValue } = resolvePaletteProps(...);

[jtmthf]

@diegohaz I recently had a use for this project and I forgot that I still had this PR open. I applied your suggestions and cleaned a few things up if you're still interested in this feature.

[exports]

I'm looking forward the new feature to solve my recent problems, will this be merged?

[iamgbayer]

is there something else needed to merge this pr?