Get to version 1

This serves as a checklist of what we need to do to get to version 1.

- [ ] implement all standard components we're aware of
- [ ] example site should show usage of all exported components
- [ ] all form components and validation should be working with final form in example site as expected
- [ ] collaborator review of all markup/props used by example site
- [x] storybook review
- [x] generate list of all exported components + props (api documentation)
- [x] working usage in an internal tool
- [x] ~decision on grid tooling~ [decided](https://github.com/penx/govuk-react/issues/61#issuecomment-368578006)
- [x] API documentation #64
- [ ] Form fields and inputs have a naming convention, documented in the readme, and follow the same prop structure
- [x] asAnchor pattern is consistent, working and compared with how other libraries manage this
- [ ] a common approach for active links (as per react-router NavLink) decided

Conventions are consistent and present in README:
- white space
- anchors
- form fields vs. inputs