filled background

@drsnyder 
Adding support for the ?fill=<color> query parameter, which supports [ImageMagick colors](http://www.imagemagick.org/script/color.php), including "transparent", which will convert the image to png format.

Note that this was branched from https://github.com/Wikia/vignette/pull/11, since that also touched route code and I wanted to avoid conflicts. 
If went with the set version for `q-opts-map` this might be

```
(defn query-param->thumbnail-argument
  [opt-key]
  (and (query-options opt-key) (name opt-key)))
```

To convert something like `:fill` to `"fill"` only if it exists as a valid parameter.