Adding two new social options, Pexels (a photo sharing site) and Write.as (a blogging service). Also correcting the URL for Unsplash.

And, though I chose not to change it in this PR, I do want to note that the Mastodon integration is a bit misleading. It's specific to the instance mstdn.io and doesn't have any logic to try to accommodate folks who "live" elsewhere in the fediverse.

You can see the Write.as integration live on my site - https://gallant.dev

I tested Pexels as well, and may end up adding it to my site at some point.

Thanks!