Add simple checkboxes on general options page to disable membership plugin stylesheets

In the newer versions of membership, it is not straight forward (I can't figure it out) to deregister membership plugin stylesheets.

This used to be possible using standard WP deregister code, but the way they are called in the plugin now doesn't allow this to work.

Please make simple checkboxes for each stylesheet to be disabled if we choose to.

The included stylesheets in the membership plugin are outdated and would rarely suit anyone's themes in my opinion.