Added Hover Functionality

I've amended only the necessary files for this (it's my first ever
fork/pull request on GitHub and was unsure if etiquette says I should
amend all files in the repository or just the ones to get the
functionality working).

I've added an extra option to the defaults to enable to user to decide
if they want the hover image to display on an already checked item
(enabled by default).

I've amended the small checkbox and radio button image with a green
highlighted representative of each style.

Any modifications made to the code file are marked with comments in the
following fashion: "// MrBearding Mod Start ---"