plugins: adds new no-op plugin that may be used as a test-double

Plugin can be configured to record all inputs and to respond with
arbitrary stdout or error message.  Will support upcoming integration
testing.

testutils/bad_reader.go

@@ -16,6 +16,7 @@ package testutils
 
 import "errors"
 
+// BadReader is an io.Reader which always errors
 type BadReader struct {
 	Error error
 }