Add progress container mixin

[bobbijvoet]

I needed to add some extra styles to the progress container element. Then I discovered that unlike a lot of other paper elements, the paper-progress did not have a mixin for applying styles to the container element. This PR adds that mixin.

[googlebot]

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[bobbijvoet]

I accidentally used a different email address for the commit. Added this email address to my account, so should be ok now.

[googlebot]

CLAs look good, thanks!

[keanulee]

@bobbijvoet Do you have an example of what you need to style in the progress container element? Perhaps there's another way from the outside?

[bobbijvoet]

I need an inset shadow on the progress container to show an inner border. I tried several ways but didn't succeed to reach the inner div. I looked at other paper elements and noticed that most of them actually have mixins for their containers, to follow that rule it should be added to progress as well.

[keanulee]

LGTM. Thanks @bobbijvoet

[bobbijvoet]

Great! Many thanks @keanulee