Ignore /var in output

This will serve as storage for other stuff, so we can serve something
else than the blog.

There cannot be a .gitignore in output/, since that directory gets
re-generated as a whole. (I could probably convince Pelican to always
put it there, but that seems more fragile than to hardcode it in repo
root.)

.gitignore

@@ -2,3 +2,5 @@ __pycache__/
 env/
 .*
 !.gitignore
+
+/output/var