[exp] clear RN packager cache before publish

When you publish your project, you might use different environment variables than the ones used when developing.

If you don'\''t clear the RN packager cache, you might publish to Expo production the Js bundle that is suited for your local development, and former env variable values (suited for local dev) are cached.

See related issue: expo/create-react-native-app#500

Closes expo/exp#88

dev/exp/src/commands/publish.js

@@ -31,7 +31,7 @@ export async function action(projectDir: string, options: Options = {}) {
   if (status !== 'running') {
     log('Unable to find an existing exp instance for this directory, starting a new one...');
     installExitHooks(projectDir);
-    await Project.startAsync(projectDir, {}, !options.quiet);
+    await Project.startAsync(projectDir, {reset: options.clear}, !options.quiet);
     startedOurOwn = true;
   }
 
@@ -81,6 +81,7 @@ export default (program: any) => {
     .description('Publishes your project to exp.host')
     .option('-q, --quiet', 'Suppress verbose output from the React Native packager.')
     .option('-s, --send-to [dest]', 'A phone number or e-mail address to send a link to')
+    .option('-c, --clear', 'Clear the React Native packager cache')
     .option(
       '-c --release-channel <release channel>',
       "The release channel to publish to. Default is 'default'.",