rewriteExt option added to DynamicAssets; potential bug with windows paths fixed

I can't believe I didn't notice this earlier. There is a pretty big issue with `DynamicAssets`.
eg:
```coffee
new DynamicAssets
  type: StylusAsset
  urlPrefix: '/css'
  dirname: '/styl'
```
This serves all the compiled stylus files with the `.styl` extension **not** the `.css` extension.
`/css/filename.styl` when it should be `/css/filename.css`
So I added the rewriteExt option; probably not be the best solution but thats all I could think of.
```coffee
new DynamicAssets
  type: StylusAsset
  urlPrefix: '/css'
  dirname: '/styl'
  rewriteExt: 'css' # <- addition
```

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I also fixed a potential problem with building the url directly with the ```file.relpath``` on windows.