dataset

dataset is one of those annoying, impossible to polyfill html5 features. o-dom is potentially a good place to put a method `.dataset(el)` that gets dataset if present natively, but fetches from attributes if not. A few questions remain before putting something final in
1. Should it implement setting as well as getting by setting `data-` attributes on the element
2. Should it persist the object it gets in some way or do a fresh get each time
3. How much th efunctionality from this alternative shim should we include https://github.com/code42day/dataset/blob/master/index.js
4. Do we care about getting/setting individual properties or just the entire dataset object

@triblondon @dansearle-ft