Custom JS lint rules are sometimes ignored

When coding in anything other than CoffeeScript or IcedCoffeeScript, JS lint rules specified in `mimosa-config.coffee` get ignored.

Since mimosa doesn't provide default options for those other extensions (`@defaultOptions[options.extension]` is `undefined`), `rules` gets assigned a value of `undefined`, even when `@options` is not:

```coffeescript
rules = if @defaultOptions[options.extension]
  _.extend({}, @defaultOptions[options.extension], @options)
```