New Option `startbymousedown` and options by data-attributes

i made some changed to your plugin. If yiu want you can merge them.

- added option `startbymousedown` to start zoom on mousedown instead of hover
- added data-attributes handling to set the options by a data attribute
- changed the options to lowercase so that they can be set by data attributes
- added outside check by okj579
- updated demo/docs page