ThemeEditor public apply method needs null check

[ross-holloway94]

The Readme states:

The apply method optionally takes a high-order function as a argument. This serves as a callback that will be triggered once Colorful has finished it's theme changes

However, ThemeEditor#apply() infers a @nonnull annotation on both of its arguments.
If this is to match the readme, the callback should be optional, and therefore must have a null check before being called.

I believe this will also make things easier for non-Kotlin users to make use of.

Thanks