Allow changing of defaults

First off, great library. 

One use case we would like to be able to work around is setting the display format default globally. This way we don't have to remember to set the option on every individual instantiation and can just add this configuration to our existing per locale JS customization files. 

Several popular libraries (Bootstrap, Select2, etc) use this strategy so I don't think it will be too controversial of a change but let me know if you have any concerns.

Thanks!