Add SVG support?

[srushe]

Hello. I've made a small-ish update to add support for using an inline SVG as a customToggle. This fixes a javascript error I've encountered when I tried this for a site I'm working on. I'm putting this PR up in case it might be worth applying, but obviously feel free to ignore it if it doesn't fit.

The error I encountered is...

Uncaught TypeError: el.className.indexOf is not a function
    addClass http://localhost:1313/j/responsive-nav.js:152
    open http://localhost:1313/j/responsive-nav.js:301
    toggle http://localhost:1313/j/responsive-nav.js:286
    _onTouchEnd http://localhost:1313/j/responsive-nav.js:584
    handleEvent http://localhost:1313/j/responsive-nav.js:386
    addEvent http://localhost:1313/j/responsive-nav.js:56
    _init http://localhost:1313/j/responsive-nav.js:432
    ResponsiveNav http://localhost:1313/j/responsive-nav.js:246
    responsiveNav http://localhost:1313/j/responsive-nav.js:653
    <anonymous> http://localhost:1313/:1381

The documentation on MDN regarding className mentions that if the element is an SVG then className returns an instance of SVGAnimatedString, rather than a string, which causes the error.

I've updated responsive-nav.js to use getAttribute to determine the class name, as well as using setAttribute to set the new class name, as recommended on MDN.