[ProgressIndicator] Closes #2361 - Allows extending LinearProgressIndicator and CircularProgressIndicator

[ekager]

Thanks for starting a pull request on Material Components!

Don't forget:

• Identify the component the PR relates to in brackets in the title.
  [Buttons] Updated documentation
• Link to GitHub issues it solves. closes #1234
• Sign the CLA bot. You can do this once the pull request is opened.

Contributing
has more information and tips for a great pull request.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[paulfthomas]

@ekager can you sign the Contributor License Agreement (CLA)?

[ekager]

I believe I did after the bot commented. See checks here.