fix: reporter allow using a externally provided source cachere for reporters

change `coverageReporter` options to allow for specifying `sourceStore`, this
is an instance of `istanbul.Store` to look up source files. This allows using
external coverage collectors like `browserify-istanbul` for instrumenting code.

Author: weikinhuang

lib/reporter.js

@@ -134,11 +134,12 @@ var CoverageReporter = function(rootConfig, helper, logger) {
                                                                                            reporterConfig.dir || config.dir,
                                                                                            reporterConfig.subdir || config.subdir)));
 
-          var options = helper.merge({}, reporterConfig, {
-            dir : outputDir,
+          var options = helper.merge({
             sourceStore : new SourceCacheStore({
               sourceCache: sourceCache
             })
+          }, reporterConfig, {
+            dir : outputDir
           });
           var reporter = istanbul.Report.create(reporterConfig.type || 'html', options);