common.mk: fix clean target to remove binaries, not source directories (#183)

The clean target was using 'rm -rf ./cmd/$(APP)' which deletes the entire
source directory. Since 'go build ./cmd/$(APP)' outputs the binary to the
current directory (not into cmd/), clean should remove './$(APP)' instead.

This prevents accidental deletion of customized cmd/ files (like the
streaming example's http.go with pingPonger).

Author: raphael

common.mk

@@ -70,7 +70,7 @@ build:
 	@go build "./cmd/$(APP)" && go build "./cmd/$(APP)-cli"
 
 clean:
-	@rm -rf "./cmd/$(APP)" "./cmd/$(APP)-cli"
+	@rm -f "./$(APP)" "./$(APP)-cli"
 
 test:
 	@echo TESTING...