console.warn() in Quirks Mode

ehberger and I just spent an unfortunate amount of time re-discovering that MathQuill works poorly in Quirks Mode; among other things, subscripts are way too low, and empty `.mathquill-editable`s have no height.

It's documented in the README that MathQuill needs Standards Mode, but it's still easy to forget, it would be nice for MathQuill to check [`document.compatMode`][compatMode] and like, [`console.warn()`][console.warn] or something. (Where `console.warn()` isn't supported, `setTimeout(function() { throw 'message'; });` instead?)

[compatMode]: http://www.quirksmode.org/dom/w3c_html.html#t11
[console.warn]: https://developer.mozilla.org/en-US/docs/Web/API/console.warn#Browser_compatibility