add --agree-to-terms flag

bin/defaults.js

@@ -16,6 +16,7 @@ Flags.init({ forceSave: true }).then(function({
 }) {
     var myFlags = {};
     [
+        'agree-to-terms',
         'account-key-type',
         'server-key-type',
         'subscriber-email',
@@ -29,7 +30,7 @@ Flags.init({ forceSave: true }).then(function({
         'challenge-tls-alpn-01-xxxx',
         'challenge',
         'challenge-xxxx',
-        'challenge-http-01',
+        'challenge-http-01'
     ].forEach(function(k) {
         myFlags[k] = flagOptions[k];
     });

bin/lib/flags.js

@@ -26,6 +26,11 @@ Flags.flags = function(mconf, myOpts) {
             'search all site configs rather than by --subject or --servernames',
             'boolean'
         ],
+        'agree-to-terms': [
+            false,
+            "agree to the Let's Encrypts Subscriber Agreement and Greenlock Terms of Use",
+            'boolean'
+        ],
         subject: [
             false,
             'the "subject" (primary domain) of the certificate',