fix(init): set default filename

Since d4a06f2, the config file is set only if it exists, but `karma init` requires the filename to be set. This sets the filename to `karma.conf.js` if not present.

Closes #680
Closes #681

Author: vojtajina

lib/init.js

@@ -351,7 +351,7 @@ exports.init = function(config) {
 
   sm.process(questions, function(answers) {
     var cwd = process.cwd();
-    var configFile = config.configFile;
+    var configFile = config.configFile || 'karma.conf.js';
     var templateFile = isCoffeeFile(configFile) ? COFFEE_TPL_PATH : JS_TPL_PATH;
     var replacements = getReplacementsFromAnswers(answers, getBasePath(configFile, process.cwd()));
     var content = fs.readFileSync(templateFile).toString().replace(/%(.*)%/g, function(a, key) {