feat(init): install coffee-script automatically

if the user initialize Karma passing a .coffee file we'll install
coffee-script and put it as a dev-dependency

Closes #1152

lib/init.js

@@ -255,12 +255,17 @@ exports.init = function(config) {
   sm.process(questions, function(answers) {
     var cwd = process.cwd();
     var configFile = config.configFile || 'karma.conf.js';
-    var testMainFile = (/\.coffee$/).test(configFile) ? 'test-main.coffee' : 'test-main.js';
+    var isCoffee = path.extname(configFile) === '.coffee';
+    var testMainFile = isCoffee ? 'test-main.coffee' : 'test-main.js';
     var formatter = formatters.createForPath(configFile);
     var processedAnswers = processAnswers(answers, getBasePath(configFile, cwd), testMainFile);
     var configFilePath = path.resolve(cwd, configFile);
     var testMainFilePath = path.resolve(cwd, testMainFile);
 
+    if (isCoffee) {
+      installPackage('coffee-script');
+    }
+
     if (processedAnswers.generateTestMain) {
       formatter.writeRequirejsConfigFile(testMainFilePath);
       console.log(colorScheme.success(