add basic caching support

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

cache/memory.go

@@ -0,0 +1,27 @@
+package cache
+
+import "github.com/willnorris/go-imageproxy/data"
+
+// MemoryCache provides an in-memory Cache implementation.
+type MemoryCache struct {
+	images map[string]*data.Image
+}
+
+func NewMemoryCache() *MemoryCache {
+	return &MemoryCache{
+		make(map[string]*data.Image),
+	}
+}
+
+func (c MemoryCache) Get(u string) (*data.Image, bool) {
+	image, ok := c.images[u]
+	return image, ok
+}
+
+func (c MemoryCache) Save(image *data.Image) {
+	c.images[image.URL] = image
+}
+
+func (c MemoryCache) Delete(u string) {
+	delete(c.images, u)
+}