Allow GeolocateControl to be cancellable

Nobody has claimed this yet.

## Motivation

My particular use case involves geolocating UK users and firing the normal GeolocateControl behaviour if they're within the UK, displaying error messaging if they're outside it. Currently the `geolocate` event isn't cancellable because it merely contains the `Position` data and not any methods governing how the rest of the interaction works.

I've added a Stack Overflow question [here](https://stackoverflow.com/questions/50759799/canceling-geolocation-behaviour-in-mapboxgl) in case there's a workaround I'm missing but having looked through the source of [geolocate_control.js](https://github.com/mapbox/mapbox-gl-js/blob/master/src/ui/control/geolocate_control.js) it really doesn't seem possible at present.

## Design

`geolocate` should be a cancelable event; calling `event.preventDefault` should cause the behaviour of zooming/panning to the user position and placing the relevant marker to be cancelled. This can be accomplished by moving the firing of the `geolocate` event from [here](https://github.com/mapbox/mapbox-gl-js/blob/master/src/ui/control/geolocate_control.js#L183) to the beginning of the `_onSuccess` callback.

Start in src/ui/control/geolocate_control.js and trace the _onSuccess callback and current geolocate event firing. Move the event to the beginning of _onSuccess as described, and verify that preventDefault cancels zooming, panning, and marker placement while normal events retain their behavior.

Written by the indexing model from the issue text.