Enhanced image rendering methods

Added new methods to the native module to allow:
- captureBytes() - alternative to capture - will return a node Buffer with image data rather than saving to file.
- renderSnippet() - render a snippet of HTML rather than a remote URL.
- clipToElement() - clip the rendering to a given element (by css selector). Will work with capture() and captureBytes()

A combination of these new methods will allow a streamlined pipeline if you wish to render the contents of a locally held snippet of html and return it directly via a webserver.
This will avoid a number of unnecessary file save/loads or converting image data to Base64.