[plugin.browser.zoom] add method to modify zoom level

[rodneyrehm]

JsonWire/WebDriver does not provide a way to control the browser's zoom level. This, however, may be an interesting thing to do in "monkey testing" (or similar).

Zoom can apparently manipulated via keyboard commands as this and this suggest. To make this user friendly we'd have to maintain a map (in behaviors?) of the required keyboard commands per browser and OS.