add basic caching support

includes two implementations, a no-op NopCache and an in-memory
MemoryCache.

cache/cache.go

@@ -0,0 +1,17 @@
+// Package cache implements a image cache.
+package cache
+
+import "github.com/willnorris/go-imageproxy/data"
+
+// Cache provides a cache for image metadata and transformed variants of the
+// image.
+type Cache interface {
+	// Get retrieves the cached Image for the provided image URL.
+	Get(string) (image *data.Image, ok bool)
+
+	// Put caches the provided Image.
+	Save(*data.Image)
+
+	// Delete deletes the cached Image and all variants for the image at the specified URL.
+	Delete(string)
+}