Use HTML source if block.config.src is empty

I wanted the ability to do this: 

```
[D] Source file before processing.
[D]       <!-- fileblock:other reload -->
      <script src="//localhost:35729/livereload.js"></script>
      <!-- endfileblock -->

      <!-- build:js /assets/js/app.js -->
      <!-- fileblock:js scripts -->
      <script src="assets/js/libs/a.lib.js"></script>
      <script src="assets/js/libs/b.lib.js"></script>
      <script src="assets/js/src/**/*.js"></script>
      <!-- endfileblock -->
      <!-- endbuild -->

[D] Source file after processing.
[D]       <!-- fileblock:other reload -->
      <script src="//localhost:35729/livereload.js"></script>
      <!-- endfileblock -->

      <!-- build:js /assets/js/app.js -->
      <!-- fileblock:js scripts -->
      <script src="/assets/js/libs/a.lib.js"></script>
      <script src="/assets/js/libs/b.lib.js"></script>
      <script src="/assets/js/src/a.js"></script>
      <script src="/assets/js/src/b.js"></script>
      <!-- endfileblock -->
      <!-- endbuild -->
```

This pull request allows this.