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

Let me know what you think