feat(commands/run): introduce --client-version

This commit introduces a new option `--client-version` that can be used to
specify the Aragon client version to be used as discussed in aragon#182. A version
can be any commit-ish values (hashes, branches, tags).

Example:

```
$ aragon run --client-version=fa2025232330bc3e4d3b792cab4bf44d754d33e6
```
Or

```
$ aragon run --client-version=dev
```

Closes aragon#182

src/commands/run.js

@@ -35,7 +35,7 @@ const url = require('url')
 const TX_MIN_GAS = 10e6
 const CLIENT_PORT = 3000
 
-const aragonClientVersion = pkg.aragon.clientVersion;
+const defaultClientVersion = pkg.aragon.clientVersion;
 
 exports.command = 'run'
 
@@ -86,6 +86,9 @@ exports.builder = function (yargs) {
     description: 'Arguments for calling the app init function',
     array: true,
     default: [],
+  }).option('client-version', {
+    description: 'Version of Aragon client used to run your sandboxed app',
+    default: null
   })
 }
 
@@ -124,6 +127,7 @@ exports.handler = function ({
     httpServedFrom,
     appInit,
     appInitArgs,
+    clientVersion
   }) {
   apmOptions.ensRegistryAddress = apmOptions['ens-registry']
   const showAccounts = accounts
@@ -232,7 +236,8 @@ exports.handler = function ({
         {
           title: 'Download wrapper',
           task: (ctx, task) => {
-            const CLIENT_PATH = `${os.homedir()}/.aragon/wrapper-${aragonClientVersion}`
+            clientVersion = clientVersion || defaultClientVersion
+            const CLIENT_PATH = `${os.homedir()}/.aragon/wrapper-${clientVersion}`
             ctx.wrapperPath = CLIENT_PATH
 
             // Make sure we haven't already downloaded the wrapper
@@ -246,12 +251,10 @@ exports.handler = function ({
             fs.ensureDirSync(CLIENT_PATH)
 
             // Clone wrapper
-            return clone(
-              'https://github.com/aragon/aragon',
-              CLIENT_PATH,
-              { checkout: aragonClientVersion }
-            )
-          },
+            return clone('https://github.com/aragon/aragon', CLIENT_PATH, {
+              checkout: clientVersion
+            })
+          }
         },
         {
           title: 'Install wrapper dependencies',