feat(runner): Return emitter that forwards server data

Allow the consumer to decide whether to echo the runner progress or not.

This might be the case when Karma.run() is called inside the same
process/terminal as the server is running in (e.g. development
servers).

Fixes karma-runner#2121, karma-runner#2799

docs/dev/04-public-api.md

@@ -129,6 +129,17 @@ runner.run({port: 9876}, function(exitCode) {
 })
 ```
 
+`runner.run()` returns an `EventEmitter` which emits a `progress` event passing
+the reporter output as a `Buffer` object.
+
+You may listen for that event to print the reporter output to the console:
+
+```javascript
+runner.run({port: 9876}).on('progress', function(data) {
+  process.stdout.write(data)
+})
+```
+
 ## karma.stopper
 
 ### **stopper.stop(options, [callback=process.exit])**

lib/runner.js

@@ -3,6 +3,7 @@
 const http = require('http')
 
 const constant = require('./constants')
+const EventEmitter = require('events').EventEmitter
 const helper = require('./helper')
 const cfg = require('./config')
 const logger = require('./logger')
@@ -40,6 +41,7 @@ function run (config, done) {
   config = cfg.parseConfig(config.configFile, config)
 
   let exitCode = 1
+  let emitter = new EventEmitter()
   const options = {
     hostname: config.hostname,
     path: config.urlRoot + 'run',
@@ -54,6 +56,7 @@ function run (config, done) {
     response.on('data', function (buffer) {
       const parsedResult = parseExitCode(buffer, exitCode, config.failOnEmptyTestSuite)
       exitCode = parsedResult.exitCode
+      emitter.emit('progress', parsedResult.buffer)
       process.stdout.write(parsedResult.buffer)
     })
 
@@ -77,6 +80,8 @@ function run (config, done) {
     refresh: config.refresh,
     colors: config.colors
   }))
+
+  return emitter
 }
 
 exports.run = run