'element alias' registration

[nitzanvolman]

Create the ability to register elements as aliases to other elements.
Allows code re-use when a required tag functionality already exists, but required for a different tag.
Specific use case is amp-taboola which derives it's functionality from amp-ad but requires a different name.
separating this PR from the amp-taboola PR to allow separate discussions.

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[ampsauce]

Tests failed. Automated cross-browser testing via Sauce Labs and Travis CI shows that the JavaScript changes in this pull request are: BUSTED

Commit: fc8515d
Build details: https://travis-ci.org/ampsauce/amphtml/builds/103981120

(Please note that this is a fully automated comment.)

[nitzanvolman]

the functionality here is covered in #1607, making this PR obsolete