Add a public API to return the definition for a style

[kevinbarabash]

I was experimenting with Storybook's jsx addon. Here's its output for a component that's passed an aphrodite style as a prop:

I would like to update the plugin to handle aphrodite styles more gracefully by being display the definition for the rule instead of the aphrodite internals.

Proposed API:

function getStyleDefinition(style) => Object

[lencioni]

To clarify, you would expect this to return the value of the _definition key?

[kevinbarabash]

Yes. But we might change how we store the definition in the future. This might make sense as a separate package inside this repo.

[lencioni]

I think I feel similar to https://github.com/Khan/aphrodite/issues/306#issuecomment-373825834 about this