Add support for preview images for WebSocket runtimes

If type member of a network output message is set to "previewurl",
we show the url as an image in the preview pane.
Also renames "console" DOM element to not override JS console API.
Required for jonnor/imgflo#5
Merge pull request #1 from jonnor/imgflo-preview

Add support for preview images for WebSocket runtimes