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>
```

hugolib/hugo_sites.go

@@ -120,12 +120,14 @@ func (h *HugoSites) getNodes(nodeID string) Nodes {
 	return Nodes{}
 }
 
-// Reset resets the sites, making it ready for a full rebuild.
+// Reset resets the sites and template caches, making it ready for a full rebuild.
 func (h *HugoSites) reset() {
 	h.nodeMap = make(map[string]Nodes)
 	for i, s := range h.Sites {
 		h.Sites[i] = s.reset()
 	}
+
+	tpl.ResetCaches()
 }
 
 func (h *HugoSites) reCreateFromConfig() error {