generated es5-loader

[valdrinkoshi]

Fixes #737 by generating webcomponents-es5-loader.js out of the custom elements native shim + webcomponents-loader.
This allows browsers that implement customElements (e.g. chrome, STP) to use es5 style js to define custom elements.
The gambit is: only the native-shim is needed synchronously when the script is executed. We bundle it into the loader for this reason.
This requires users to NOT compile webcomponents-es5-loader.js ⚠️

Here's an example of it working as expected http://jsbin.com/babifij/2/edit?html,console,output

Fixes #722 by renaming webcomponents-loader to webcomponents-es6-loader

[dfreedm]

I like this approach. Let's also try #722 out in this PR.

[valdrinkoshi]

@azakus ready for review! Firefox 52 timed out for some reason (can't repro locally), so I've kicked the build again https://travis-ci.org/webcomponents/webcomponentsjs/builds/213528757

Update: yay the build is green 💚

[dfreedm]

Closing in favor of #741