Material-ui: Allow any wanted color in CircularProgress

[frizzkitten]

It's really annoying that I can no longer use the color prop to give CircularProgress whatever color I want. That's unnecessarily constricting. You can allow "primary" or "secondary" like you have it if you want but don't make me choose between only those two colors.

• [x] This is a v1.x issue.
• [x] I have searched the issues of this repository and believe that this is not a duplicate.

[henkvhest]

You have multiple options to modify the color:
https://codesandbox.io/s/x2942qwrvw

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