Added ability to enable each theme individually

Hi Kolin,

Thanks for making it happen! I’ve wanted to try this way of syntax coloring ever since I saw that post.

I added ability to enable each theme individually, to avoid interfering with existing themes, which (especially the dark ones) don’t work well with Polychromatic.

- Polychromatic themes are enabled by default.
- Other themes need to be enabled individually.
- Polychromatic can still be disabled globally. (Should this setting be removed, though?)

What do you think?

—Anton