A simple plugin that verify if an element is in the viewport.
##options offset: values between 0 and 1 (default: 0) - the element visible area in the viewport
##events onEnter: (function) callback triggered when the element enter in the viewport.
onExit: (function) callback triggered when the element leaves the viewport.
onceEnter: (function) callback triggered when the element enter in the viewport just in the first time.
onceExit: (function) callback triggered when the element leaves the viewport just in the first time.