Add simple in-memory cache (#2231)

* Add simple in-memory cache

* Support configuring cache expiration duration

* Support preinitializing cache with items

* Register cache

cache/options.go

@@ -0,0 +1,40 @@
+package cache
+
+import "time"
+
+// Options represents the options for the cache.
+type Options struct {
+	Expiration time.Duration
+	Items      map[string]Item
+}
+
+// Option manipulates the Options passed.
+type Option func(o *Options)
+
+// Expiration sets the duration for items stored in the cache to expire.
+func Expiration(d time.Duration) Option {
+	return func(o *Options) {
+		o.Expiration = d
+	}
+}
+
+// Items initializes the cache with preconfigured items.
+func Items(i map[string]Item) Option {
+	return func(o *Options) {
+		o.Items = i
+	}
+}
+
+// NewOptions returns a new options struct.
+func NewOptions(opts ...Option) Options {
+	options := Options{
+		Expiration: DefaultExpiration,
+		Items:      make(map[string]Item),
+	}
+
+	for _, o := range opts {
+		o(&options)
+	}
+
+	return options
+}