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>
```

docs/content/templates/functions.md

@@ -356,7 +356,7 @@ e.g.
        {{ .Content }}
     {{ end }}
 
-## Files    
+## Files
 
 ### readDir
 
@@ -372,6 +372,16 @@ Reads a file from disk and converts it into a string. Note that the filename mus
 
  `{{readFile "README.txt"}}` → `"Hugo Rocks!"`
 
+### imageConfig
+Parses the image and returns the height, width and color model.
+
+e.g.
+```
+{{ with (imageConfig "favicon.ico") }}
+favicon.ico: {{.Width}} x {{.Height}}
+{{ end }}
+```
+
 ## Math
 
 <table class="table table-bordered">