Revamp template polyfill (#813)
Revamp template polyfill (#813)

Does a few things:

- Minimal poyfills no longer collide
  - Since lit doesn't create a HTMLTemplateElement class, checking for its existence doesn't prevent a second polyfilling (imagine two lit-html versions being included on the page)
- Makes `content` non-writable
- Makes `innerHTML` enumerable (Element class methods/accessors are, strangely, all enumerable)
- Reuses the original `template` element when upgrading it, instead of creating a div

lit/lit,Polymer/lit-html,Polymer/lit-html,Polymer/lit-html,lit/lit,Polymer/lit-html,lit/lit,lit/lit