Build linux, darwin and windows binaries

Makefile

@@ -1,5 +1,19 @@
+.PHONY: test clean bin
+
 bin/assume-role: *.go
-	go build -o bin/assume-role .
+	go build -o $@ .
+
+bin: bin/assume-role-Linux bin/assume-role-Darwin bin/assume-role-Windows.exe 
+
+bin/assume-role-Linux: *.go
+	env GOOS=linux go build -o $@ .
+bin/assume-role-Darwin: *.go
+	env GOOS=darwin go build -o $@ .
+bin/assume-role-Windows.exe: *.go
+	env GOOS=windows go build -o $@ .
+
+clean:
+	rm -rf bin/*
 
 test:
 	go test -race $(shell go list ./... | grep -v /vendor/)