Added an option for customizing the close button text/symbol (see #10)
added a CSS classname ("cookiebanner-close") to the element containing the close button (which should further help customizations)
fixed 'data-' options handling which wasn't really working as it should've been (on browsers implementing dataset)
switched to using camelCase option names within the code (due to the dataset API converting automatically and it being easier to replicate for browsers that don't support dataset)
avoid "polluting" the global namespace and instantiating Cookiebanner if we're not being invoked in a way that really requires it
added 2 simple demos within the tests folder for easier manual testing