Make awesomplete instance awailable on input element

[vlazar]

Having Awesomplete instance available in events as this.awesompelete will handy.
Discussed here #16821 (comment)

// in all event handlers:
var me = this.awesomplete;

me.ul.children[me.index]; // selected <li> DOM node
me.suggestions[me.index]; // selected suggestion item

// and so on...

[LeaVerou]

How about a WeakMap on Awesomplete? Then to get the instance you'd do something like Awesomplete.inputs.get(element). Then we don't need to add properties to elements, which many people have a knee jerk reaction against.
Also, if we do end up making Bliss a dependency, we can add it to element._.

[vlazar]

We've discussed WeakMap before. I thought it wasn't an option for the nearest future with current support in browsers. #16821 (comment)

[LeaVerou]

As long as it's not used internally, people can always choose whether they want to use it or not and take the reduced browser support (or include a polyfill).
I’m not against having auxillary features with different browser support if it's clearly indicated in the docs.
We would, however, need to check for it before using it, to avoid breakage in environments where it's not supported.