Ensure we call done() on the async onExit to not prevent exitting aft…

…er single run. Fixes #5

lib/reporter.js

@@ -120,7 +120,17 @@ const AggregatedCoverageReporter = function(injector, logger, config, baseReport
   };
 
   this.onExit = function(done) {
-    callThrough('onExit', done);
+    // We cannot call callThrough here as we must determine when all relevant reporters have called done()
+    const promises = reporters.map(({name, reporter}) => new Promise((resolve) => {
+      if (_.isFunction(reporter.onExit)) {
+        log.debug(`relaying done() on reporter:${name}`);
+        reporter.onExit(() => resolve());
+      } else {
+        resolve();
+      }
+    }));
+
+    Promise.all(promises).then(() => done());
   };
 };