feat(slider): Implement continuous slider

[pgbross]

Prototype implementation of continuous slider (implementing #25)

I am working on a project that needs a continuous slider but notice the core team are not due to work on an mdc-slider component for some while. As I have created a component for my own use, I thought I would create a pull-request of the work I have so far in case it is useful enough to be incorporated earlier or if not, then it may at least be useful to the core team when they can start work on the slider component.

The continuous slider has been prototyped in Chrome, Firefox, IE, and iOS Safari (for touch awareness) and was heavily influenced by the concepts in the MDL v1 slider. It tries to follow examples and techniques from other mdc-components already in the material-components-web, but if you do wish to eventually incorporate it into the library it quite likely needs several iterations and or consultation/guidance on how to refine it and make it ready for production.

Let me know if it is useful and if you wish to take it further.

[codecov-io]

Codecov Report

Merging #487 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #487      +/-   ##
=========================================
+ Coverage   99.19%   99.2%   +<.01%     
=========================================
  Files          40      43       +3     
  Lines        1980    2130     +150     
  Branches      250     260      +10     
=========================================
+ Hits         1964    2113     +149     
- Misses         16      17       +1

Impacted Files	Coverage Δ
packages/mdc-slider/continuous/foundation.js	100% <100%> (ø)
packages/mdc-slider/continuous/constants.js	100% <100%> (ø)
packages/mdc-slider/continuous/index.js	100% <100%> (ø)
packages/mdc-ripple/foundation.js	99.48% <0%> (-0.52%)	⬇️

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[traviskaufman]

Hey @pgbross,

Thanks for taking the time to share your code with us and do all of this work. While we'd be more than happy to assist you in taking this on, I feel that the best way to do this would be by closing this PR and tracking development on the Github Issue itself. This way, we can guide you through the correct eng process for developing new components, provide you with some visual prototypes (which we have worked on for sliders already), and ensure that the DOM structure and API is what we expect without having to go through numerous rounds of PR reviews. Thus, I'm going to close this for now.

If you'd like to take a pass at sliders, please comment on the issue and we'll get the ball rolling on helping you with an initial visual prototype / eng outline. And thanks again for your willingness to contribute and make our codebase that much better!