Tell docs.rs to use dox feature

Just learned about docs.rs which automatically builds docs for crates.
Tell it to build i3ipc with the "dox" feature, which it should listen
to thanks to onur/docs.rs#73.

Cargo.toml

@@ -18,3 +18,6 @@ i3-4-13 = ["i3-4-12"]
 i3-4-14 = ["i3-4-13"]
 i3-next = ["i3-4-14"]
 dox = ["i3-next"]
+
+[package.metadata.docs.rs]
+features = ["dox"]