WIP: Changes for a new package view system

### TODO

* [x] Move the ViewRegistry to `atom.views`
* [x] Add HTMLRequire
* [x] Add the polymer stuff to core
* [x] Publish `elmer` https://github.com/atom/elmer
* [x] Put the template system through the ringer. Does it do all we need / want? https://github.com/benogle/template-explore
* [ ] Move templating stuff out of atom and into its own file
* [ ] Add a couple global template filters (pluralize, classList)
* [ ] Document `ViewRegistry` 
* [ ] Document `HTMLRequire` 

Closes #3977 
Closes #3752