New: enable/disableScrollable public functions now completely disable/enable viewer scrolling

enable/disableScrollable public functions now work on all scrolling events by toggling CSS overflow: auto/hidden on the outer div.

Also, enableKeyScroll documentation has been updated to reflect the fact that when enableKeyScroll is on, it will capture all key-scrolls in the entire window, and when off, scrolling behaves in accordance with browser defaults.

Closes #172
Docs: update settings documentation URL