Adds an option that forces a scroll lock on an element.

I ran into an issue with the latest version of __Google Chrome 39.0.2171.71 (Official Build)__ on __OS X 10.9.5 (13F34)__ with __Show Scroll__ Bars set to __When Scrolling__ in the General section of System Preferences. For some reason Google Chrome treats the width of elements differently based on whether the element includes a scroll bar or not. This means that if a user has this option in OS X enabled, the `hasVerticalScroll()` function will always return false, ensuring the plugin fails to apply a scroll lock to the element. I'm not sure if this is a bug with Google Chrome or intended behaviour but either way it looks like your plugin won't work in this situation. 

I've made a few adjustments to suit but I haven't updated the minified version of the plugin and didn't generate the sourcemap. It might be worth adding your Grunt configuration to the repository and creating a set of contributing guidelines to make it easier for future contributors.

I'm happy to have a look at making this pull request easier to merge by finishing it off tonight if you can supply your Gruntfile.
if its forced, then no need to call `hasVerticalScroll`, this should be  `if (force === true || hasVerticalScroll($this))`