Add ability to enable only form_change, or form_submit, focus_form or any combination (close #371)

[matus-tomlein]

Issue #371

This PR adds ability to configure which of the three types of form events to track. It adds an option to the enableFormTracking call to list the events to be tracked.

API

enableFormTracking({
    options: {
        events: ['submit_form', 'focus_form', 'change_form']
    },
});

The events option is optional and, if not present, all three events are tracked.

Docs

Here is a PR in data-value-resources repo for the documentation changes.

Implementation

The list of subscribed events is checked in addFormListeners before adding event listeners to form fields (for focus and change) and the form (for submit).

I added an end-to-end test that checks that change events are not tracked if not subscribed to.

Also tried it out in a demo ReactJS app, but that's not part of this PR.

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[greg-el]

LGTM! Tiny nitpick about the test description, but the code seems fine to me.

[greg-el]

nitpick: duplicate 'events' in the test desc:
change_form events events when focus_form