Extract styles from more inputs

[dylanslewis]

There are more tools emerging for creating a design system in a separate app to Sketch, so stylesync should become more versatile in the type of inputs it can handle, including:

• Lona
• InVision DSM
• Figma

Lona stores styles as simple JSON files, so it should be pretty easy to parse all the information. It may even be worth raising a PR to Lona once this is implemented, since its code generation feature is still very nascent.