Add clean target (#161)

Adds a `make clean` target that invokes `cargo clean`.

Makefile

@@ -51,6 +51,10 @@ fetch: Cargo.lock
 .PHONY: build
 build: $(TARGET_BIN)
 
+.PHONY: clean
+clean:
+	$(CARGO) clean --target-dir $(TARGET)
+
 .PHONY: test
 test: fetch
 	$(CARGO_TEST) --no-default-features

README.md

@@ -36,6 +36,7 @@ A `Makefile` is provided to automate most build tasks. It provides the
 following targets:
 
 * `make build` -- Compiles the proxy on your local system using `cargo`
+* `make clean` -- Cleans the build target on the local system using `cargo clean`
 * `make test` -- Runs unit and integration tests on your local system using `cargo`
 * `make test-flakey` -- Runs _all_ tests, including those that may fail spuriously
 * `make package` -- Builds a tarball at