Add .style.yapf config that matches the Google internal style guide.

Unfortunately the Google internal formating is different from PEP8
(column limit 79 -> 80 and indent width 4 -> 2, a few things around when
and how to split lines). YAPFs own style matches this.

YAPF will automatically pick up the config when run anywhere inside the
repository.

.style.yapf

@@ -0,0 +1,2 @@
+[style]
+based_on_style: yapf