Add support to tap-drag for zoom-in, zoom-out

[nvkelso]

Description

In Eraser Map we allow a tap-up-tap-drag behavior to zoom the map in and out – this is especially useful for 1 handed operation of the map (for pinch zoom it requires 1 hand to hold, 1 hand to pinch).

[nvkelso]

Related:

• Basic zoom in: https://github.com/mapzen/ios/issues/321
• Basic zoom out: https://github.com/mapzen/ios/issues/323

[matteblair]

We have this feature on Android because it is provided in a built-in gesture detector (they call it "quick zoom") and we only had to set a flag to turn it on. If I remember, this is not built-in on iOS, so we would have to write custom gesture recognizer code for it. I'm also not sure if this is a common gesture paradigm on iOS the same way it is on Android - an iOS user would probably know better than I would.