Add clippy to pre-commit githook

We have a `pre-commit` githook, add to it a call to `cargo clippy`.

Author: tcharding

githooks/pre-commit

@@ -43,4 +43,7 @@ EOF
 fi
 
 # If there are whitespace errors, print the offending file names and fail.
-exec git diff-index --check --cached $against --
+git diff-index --check --cached $against -- || exit 1
+
+# Check that code lints cleanly.
+cargo clippy --all-features -- -D warnings || exit 1