tpl: Add imageConfig function

tpl: Add imageConfig function

Add imageConfig function which calls image.DecodeConfig and returns the height, width and color mode of the image. (#2677)

This allows for more advanced image shortcodes and templates such as those required by AMP.

layouts/shortcodes/amp-img.html
```
{{ $src := .Get "src" }}
{{ $config := imageConfig (printf "/static/%s" $src) }}

<amp-img src="{{$src}}"
           height="{{$config.Height}}"
           width="{{$config.Width}}"
           layout="responsive">
</amp-img>
```