ES6 compiler testing pages

This changes build.js so that in addition to outputting `es6/index.html`, it additionally compiles the ES6 tests' code using various ES6 compilers, and creates HTML test pages using the output, in the `es6/compilers/` directory. These pages are "auxiliary" pages, comparable to the ES5 strict mode page, and thus only contain the current browser column.

These test pages are not yet public-facing (they aren't linked from `es6/index.html`) but have been cleaned up to be mostly presentable.

The test pages are primarily designed for gleaning compiler results easily. If you wish to record results based on their browser response, take care to view them in IE 10, or another browser with low native ES6 support (and subtract from the results the few features that the browser does support natively).