Allow user to supply via attribute whether they want the indicator enabled

Fixes #21 (or so I hope)

Hi there

`indicator="disabled"` in this PR will allow the user to declare whether they want the indicator animating or not
Ideally, the indicator element itself would be hidden entirely (this is not the case)

I wasn't entirely sure how to expose an attribute/property that a user could call externally (via inspect &  `$0.indicator` or otherwise), and the code here looks quite different than https://developer.mozilla.org/en-US/Apps/Tools_and_frameworks/Web_components so I figured I'd start this PR to start the conversation

Let me know what you think
I guess this is the part of the spec I don't really understand from looking at various docs.
I've tried adding a `set: function(newVal) { console.log(newVal); }` but never see any output when I meddle with the DOM node via inspector, nor if I do `$0.setAttribute('noindicator', 'foo')`
Is there a good way/place to add a listener or something s.t. the component can react when a user makes changes?