Add support for custom log reporter cli option for local server

Summary:
In Expo tools such as XDE, exp we listen listen to stdout from the packager process and print it in our own buffer. In the case of XDE, an electron app, our log pane is DOM-based, and before printing each log chunk we need to remove special tty characters and sometimes parse it to get information that we need (eg: progress bar). By using a custom reporter, we can take the raw events and pass them along in a format that is easy to consume by XDE and exp. This same motivation applies to create-react-native-app, where we currently don't show a progress bar in the terminal, but we can with this change.

Create `LogReporter.js` in the root of a project with the CLI changes included in this PR.

```
class LogReporter {
  update(event) {
    console.log(JSON.stringify(event));
  }
}

module.exports = LogReporter;
```

Now, run `react-native start --customLogReporterPath=LogReporter.js` -- all of the raw events will be output as JSON (while the logs
Closes facebook/react-native#13172

Differential Revision: D4795760

Pulled By: hramos

fbshipit-source-id: 80164b2f30e33a3f9965f4865a8404f8640a52c1

Author: brentvatne

server/runServer.js

@@ -10,7 +10,6 @@
 
 const InspectorProxy = require('./util/inspectorProxy.js');
 const ReactPackager = require('../../packager/react-packager');
-const TerminalReporter = require('../../packager/src/lib/TerminalReporter');
 
 const attachHMRServer = require('./util/attachHMRServer');
 const connect = require('connect');
@@ -86,6 +85,21 @@ function getPackagerServer(args, config) {
   const providesModuleNodeModules =
     args.providesModuleNodeModules || defaultProvidesModuleNodeModules;
 
+  let LogReporter;
+  if (args.customLogReporterPath) {
+    try {
+      // First we let require resolve it, so we can require packages in node_modules
+      // as expected. eg: require('my-package/reporter');
+      LogReporter = require(args.customLogReporterPath);
+    } catch(e) {
+      // If that doesn't work, then we next try relative to the cwd, eg:
+      // require('./reporter');
+      LogReporter = require(path.resolve(args.customLogReporterPath));
+    }
+  } else {
+    LogReporter = require('../../packager/src/lib/TerminalReporter');
+  }
+
   return ReactPackager.createServer({
     assetExts: defaultAssetExts.concat(args.assetExts),
     blacklistRE: config.getBlacklistRE(),
@@ -96,7 +110,7 @@ function getPackagerServer(args, config) {
     platforms: defaultPlatforms.concat(args.platforms),
     projectRoots: args.projectRoots,
     providesModuleNodeModules: providesModuleNodeModules,
-    reporter: new TerminalReporter(),
+    reporter: new LogReporter(),
     resetCache: args.resetCache,
     transformModulePath: transformModulePath,
     verbose: args.verbose,

server/server.js

@@ -118,6 +118,9 @@ module.exports = {
   }, {
     command: '--reset-cache, --resetCache',
     description: 'Removes cached files',
+  }, {
+    command: '--custom-log-reporter-path, --customLogReporterPath [string]',
+    description: 'Path to a JavaScript file that exports a log reporter as a replacement for TerminalReporter',
   }, {
     command: '--verbose',
     description: 'Enables logging',