make mime types configurable

I came to the point where the predefined mime types of jetty were insufficient (no woff, eof, ttf mime types).

This was the easiest way to extend the existing mime type map of the assets servlet.

I created an additional section in the yml configuration which accepts a map of mime mappings, for example:

```yml
assets:
  overrides:
    # ...
  mimeTypes:
    woff: application/font-woff
```