Add checkbox widget (closes #6)

[cdevienne]

Description

Pull request solving issue #[6].

Checklist for new Widget

• [x] Checked out the Material design specification of the widget
• [x] Added constructors, Widgets Type and Style Type in src/Internal/[Your Widget].elm
• [x] Added styles in src/Internal/Material/[Your Widget].elm
• [x] In src/Widget.Material.elm:
  • [x] Linked each style to its representative in src/Internal/Material/[Your Widget].elm
  • [x] Added Documentation
• [x] In src/Widget.elm:
  • [x] Added a copy of the Widget Type
  • [x] Added a copy of the Style Type
  • [x] Linked each constructor to its representative in src/Internal/[You Widget].elm
  • [x] Replaced the Widget Type in the type signiture of each constructor with its definition
  • [x] Added Documentation with a small example
  • [x] run elm-verify-examples && elm-test to test the example

Optional:

• [x] Added a Page in explorer/src/Page/[Your Widget].elm and added it to explorer/src/Main.elm

[Orasund]

Thanks. Feature has be released in version 3.4.0