polymer build --base-path <some string> to set the basePath option

[oskarlh]

This adds a --base-path CLI argument, which was missing.

• CHANGELOG.md has been updated

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[oskarlh]

I signed it!

[googlebot]

CLAs look good, thanks!

[oskarlh]

I would be very grateful if someone could take a look at this 😄

[aomarks]

Thanks for the PR @oskarlh, merged!