Simplify API by only providing multiple formats

[webketje]

Would normalize the excerpt file property to always be { html: '...', text: '...' }

Pro's:

• no need for a multipleFormats option (=simpler API)
• could add support for a third format: stripped, removing only the outer <p> tags

Cons:

• not backwards-compatible
• can't just put {{ excerpt }} in a template

[webketje]

Actually this is mostly backwards-compatible, if the object is a new String or has a toString method that returns the html snippet