Use prompt instead of argv arguments in scripts

This makes it easier to handle truffle arguments which we for example
need to specify the network.
So we ask the user for input instead on using the argv array which might
change.

scripts/cli.js

@@ -1,17 +1,15 @@
 const REPL = require('repl');
+const promptly = require('promptly');
 
 module.exports = function(callback) {
   const Registry = artifacts.require('./Registry.sol');
   Registry.deployed().then(async (registry) => {
-    let contractName = process.argv[4];
-    let method = process.argv[5];
-    let args = process.argv.slice(6);
-
-    if (!contractName) {
-      console.log("Usage:");
-      console.log("  truffle exec scripts/cli.js <Contract name> <method to call> [<optional> <arguments>]");
-      callback();
-      return;
+    let contractName = await promptly.prompt('Contract Name: ');
+    let method = await promptly.prompt('Function: ');
+    let argumentInput = await promptly.prompt('Arguments (comma separated): ', { default: '' });
+    let args = [];
+    if (argumentInput !== '') {
+      args = argumentInput.split(',').map(a => a.trim());
     }
 
     let contractAddress = await registry.getProxyFor(contractName);