Emit valid event from Vue component when user updates input

[evanwills]

We need to show an error message if the user has not made a selection or the input returned an empty search.

This change adds an extra matchCount property to the autocompleteCore class which stores the number of items matched by the input

matchCount is set to one when the user makes a selection using a valid option.

The vue component, in turn emits a 'valid' event that contains an object with two properties:

• isValid {boolean} - whether or not the current state is valid
• matchCount {number} - items returned by the search, or 1 if user has made a selection.

The valid is emitted on every key stroke, submit & change and can be used by the client code to show/hide error messages.