cargo: Add some cargo configuration

Add my default clippy config and the crates.io staging registry.

.cargo/config

@@ -0,0 +1,10 @@
+[target.'cfg(feature = "cargo-clippy")']
+rustflags = [
+    "-Dwarnings",
+    "-Dclippy::pedantic",
+    "-Dclippy::nursery",
+    "-Dclippy::cargo",
+]
+
+[registries]
+staging = { index = "https://github.com/rust-lang/staging.crates.io-index" }