Ability for Tribute to work as autocomplete 🕺

[MrRobz]

These changes allow Tribute to work as an autocomplete type-head that gives suggestions as you type.

How to make tribute work as autocomplete:
passautocompleteMode: true with the rest of the Tribute configs

some points to note:

1. autocomplete mode currently does not work with collections as there is no multiple trigger points
2. when mention text is empty around cursor, it shows all the menu item. (maybe we could passing another option: shouldShowResultsForNull)

[mrsweaters]

Everything seems to be working as intended and the code looks good! Thanks for the great work!