Add support for as-you-type autocomplete

[lexicalunit]

As it's currently implemented, there's no evident event hook that could be used to support an autocomplete feature. Initially I tried to use didChangeQuery(). I got the TextEditor model out of the SelectList via selectList.refs.queryEditor, then I intended to do something like:

item = whatever() // the item we want to autocomplete to
editor.setText(item.title)
editor.selectLeft(item.title.length - query.length)

But of course using editor.setText() here triggers didChangeQuery() and causes an infinite loop. More than that though, it also triggers a filter process for the autocompleted item's text :(

I tried to work around it by using some state to make the next didChangeQuery() and filter() calls into no-ops after doing an autocompletion but it was really janky and slow and I just couldn't get it to work right. This seems like it would be better implemented within atom-select-list somehow.

[lexicalunit]

Note that using the setQuery() function from #13 , like the example code above, also triggers didChangeQuery() and only supports automatically selecting the entire query. Whereas in typical autocompletion schemes it is only autocompleted text that is selected. For example: