Allows for multiple content types
Numerous enhancements. Breaks backwards compatability.

* [Added iframe lazy loading
support.](https://github.com/cferdinandi/jellyfish/issues/3)
* Renamed `data-options` attribute to `data-load-attributes`.
* [Added options overrides on a content-by-content
basis.](https://github.com/cferdinandi/jellyfish/issues/3)
* Added callbacks during icon loading phase.
* Renamed various private and public functions to account increased
types of content that can be loaded.
Updated readme
Merge branch 'enhancements-2'