Render only single shape

[lukasmartinelli]

@amishas157 this branch was used for https://github.com/mapbox/cartoshapes.

This PR:

• Changes the whole options format (which means will be a breaking version change)
• Allows rendering only an overlay without a background
• Adds a black style which is only the black feature outline
• Adds option to specify overlay stylesheet in CLI
• Move render params guessing into a renderparams module
• Refactor renderThumbnail into renderOverlayWithBackground and renderOverlay
• One can also pipe in a feature from stdin now when doing echo '{}' | render-thumbnail - mythumb.png

[coveralls]

Coverage decreased (-4.9%) to 82.564% when pulling ec03029 on render-only-single-shape into b7327f6 on master.