Enhanced formatters

[rsimon]

The formatter API should be upgraded to the same level as in Annotorious. Instead of just allowing a single function on init, which can only output a CSS class name, we should also include:

• the ability to return CSS style rules which get embedded into the SPANs directly (i.e. no need for a "detour" via CSS class)
• the ability to inject data-* attributes
• possibly the ability to append additional markup to implement labels (like the Annotorious Shape Label plugin does)
• support for multiple formatters
• ability to add/remove formatters dynamically