websocket.js: Add optional 'options' to websocket.register

Sometimes we want an exact topic match, instead of a prefix match.
The new 'options' argument makes that possible, while also supplying a
framework for other keyword-argument style tweaks that we may want to
add in the future.
Bump version to 0.1.2

Changes since 0.1.2:
* e99dad8 websocket.js: Add optional 'options' to websocket.register