Proof-of-concept for theme props approach

[davidtheclark]

Relates to #26

The first commit on this PR does a few things:

• Modify the Button component to have a theme prop that fits the convention proposed in Extend theme* props to accept functions that receive the default class list #26: string values are appended, function values replace.
• Create an applyTheme function that we can use to apply the same convention all over the place.
• Modify the documentation to de-emphasize theme* props, since they are for advanced users, and explain them all in one place.

If we like this approach, I guess the next step would be to modify all the existing components to follow the new convention. To be clear, it would a breaking change: usage of theme* strings right now would have to adjust to accommodate the change. The intended benefit of the change is to help the most common use case and the less advanced users. If you disagree and think we should just stick with the current theme* behavior — where theme* props take a string and that string replaces the class list wholesale — please let me know! cc @tristen @mapbox/studio @frontend

Here are some shots of what the docs look like:

[davidtheclark]

Throwing this out in favor of removing theme* props, as described in #26