Use individual output directory per Asset

Hi,

For one of my projects I need to deliver `html`-files from a `bower` component. This does not work at the moment, because `html` is an "unsupported" file type. 

I'm quite sure my use case is a quite special one. Therefor I tried to solve the problem more generally by giving the user of `middleman-sprockets` the option to define an output-directory per asset:

```
sprockets.append_path 'vendor/assets/components'
sprockets.import_asset 'jquery/dist/jquery', 'individual_output_directory'
```

Would you mind to review the PR and give me feedback? :-) Thank you very much.

Cheers,
dg