optional determine literate coffee by its suffix

Add optional to treat literate coffeescript file by its suffix, so this would works too:
```
<script type="text/coffeescript" src="foobar.litcoffee"></script>
<script type="text/coffeescript" src="foobar.coffee.md"></script>
```
The same logic applying [here](https://github.com/jashkenas/coffee-script/blob/f277a43/src/command.coffee#L320).