Add default render props

[onlined]

Description

Moved the default implementation of rendering to default props,
reducing the branching. I couldn't change renderInput (custom
implementations get wrapped by a div, default one doesn't) and
renderMonth (default implementation depends on state) trivially,
so I left them out of this PR's scope.

Motivation and Context

• Simplifies the code
• Reduces branching

Checklist

[x] I have not included any built dist files (us maintainers do that prior to a new release)
[ ] I have added tests covering my changes
[x] All new and existing tests pass
[ ] My changes required the documentation to be updated
  [ ] I have updated the documentation accordingly
  [ ] I have updated the TypeScript 1.8 type definitions accordingly
  [ ] I have updated the TypeScript 2.0+ type definitions accordingly

[arqex]

Neat! Great work