Add Progress Indicators

[c-mart]

Description

Solves issue #11. Adds circular progress indicators, both indeterminate (animated spinner showing task in progress) and determinate (circular bar showing how much of a task is complete). I did not implement linear progress indicators in this PR, leaving that for another time if there is need/interest.

Checklist for new Widget

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

Optional:

• [x] Added a Material design style in src/Widget/Style/Material.elm
• [ ] Added a Ellie to the docs (+ copied it into example/src/Example/[Your Widget].elm)

[c-mart]

The example at https://material.io/develop/web/components/circular-progress relies on this external JavaScript, which we don't want. Here is someone animating the spinner with CSS. Even better, this looks like a "pure SVG" implementation using the animate and animateTransform SVG elements.

[c-mart]

@Orasund this is ready for you to review! I don't believe I can make an Ellie yet because I can't install the new changes into the Ellie environment. Please clarify if I'm wrong.

[c-mart]

@Orasund I made the requested changes, and rebased this branch against the upstream minor branch.