Add a general purpose contract CLI

This allows to call any method on any contract using the CLI.

for example:

  $ truffle exec scripts/cli.js Operator proposalsCount

scripts/cli.js

@@ -0,0 +1,34 @@
+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 contractAddress = await registry.getProxyFor(contractName);
+    console.log(`Using ${contractName} at ${contractAddress}`);
+    let contract = await artifacts.require(`./${contractName}`).at(contractAddress);
+
+    console.log(`Calling ${method} with ${JSON.stringify(args)}`);
+
+    contract[method](...args).then((result) => {
+      console.log('Result:');
+      console.log(result);
+
+      callback();
+    }).catch((error) => {
+      console.log("Call failed. Probably the contract raised an error?\n");
+      console.log("...");
+      callback(error);
+    });
+
+  })
+
+}