[Pager] Add VerticalPager

[chrisbanes]

Was surprisingly easy to refactor HorizontalPager. Most of the code changes is actually in the tests instead.

Demo

verticalpager.mp4

Closes #248

[matvei-g]

Nice!

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[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

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[chrisbanes]

@googlebot I fixed it

[matvei-g]

Thanks Chris, one comment