Add support for fling instance option

Allow to disable the fling of a scroller
Merge branch 'feature-fling-option' of git://github.com/joshfire/ftscroller into joshfire-feature-fling-option
Tweak the new option to disable the fling:
 - Rename the option from 'fling' to 'flinging' to match the other options
 - Move the flinging option check so that bouncing and other actions can still occur correctly
 - Update the Readme to document the new option