Add a script to add `themes' folder to theme path.

I have made some changes to enable loading the package from the load path.  When `require`d, it adds the `themes` directory to the `custom-theme-load-path`.

This change was mainly for integrating the package to my Emacs workflow, but I thought maybe you'd like to integrate the changes to your repository too.

(I have also made a little change to the McCarthy theme, which changes the info header to be same-as-backround text over `#0087AF` background -- which is a coldish light blue colour -- to fix the look of magit; with the vanilla theme, headers were not readable. That change is not in this changeset though, I can make another PR for that one too, if you'd like)