Add static interface implementation check

Add an additional line of code per interface to the generated mock file,
which allows the go compiler to statically check if the mock implements
the mocked interface.

main.go

@@ -2,12 +2,12 @@ package main
 
 import (
 	"bytes"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
-	"errors"
 
 	"github.com/matryer/moq/pkg/moq"
 )