Add Collections/XScreenSaver (Closes #60)

[mjturner]

This PR adds XScreenSaver, the classic Unix/X11 screensaver collection. The thumbnail was created by stitching together these screenshots. I've left it as a JPEG as the PNG was ~17MB. This closes #60.

[agarrharr]

Thanks for your submission, but I'm actually not going to add this to the list at this time.

I'lll include an excerpt from the Awesome Manifesto

The awesome manifesto

If you want your list to be included on awesome, try to only include actual awesome stuff in your list. After all, it's a curation, not a collection.

But what is awesome?

Only awesome is awesome

Research if the stuff you're including is actually awesome. Only put stuff on the list that you or another contributor can personally recommend. You should rather leave stuff out than include too much.

[mjturner]

Fair enough. IMHO, XScreenSaver should be included in any awesome list of macOS screensavers, but it's your list, so your rules.