Makefile: Add `--all-features` to clippy job (#86)

#### Problem

The clippy target does not include `--all-features`, which means we
could eventually miss warnings.

#### Summary of changes

Add `--all-features` to the target.

Makefile

@@ -33,6 +33,7 @@ spellcheck:
 clippy-%:
 	cargo $(nightly) clippy --manifest-path $(call make-path,$*)/Cargo.toml \
 	  --all-targets \
+	  --all-features \
 		-- \
 		--deny=warnings \
 		--deny=clippy::default_trait_access \