plugin: add framework to implement plugins (#580)

Fixes #485

plugin/example_test.go

@@ -0,0 +1,43 @@
+package plugin_test
+
+import (
+	"log"
+	"os"
+
+	"filippo.io/age"
+	"filippo.io/age/plugin"
+)
+
+type Recipient struct{}
+
+func (r *Recipient) Wrap(fileKey []byte) ([]*age.Stanza, error) {
+	panic("unimplemented")
+}
+
+func NewRecipient(data []byte) (*Recipient, error) {
+	return &Recipient{}, nil
+}
+
+type Identity struct{}
+
+func (i *Identity) Unwrap(s []*age.Stanza) ([]byte, error) {
+	panic("unimplemented")
+}
+
+func NewIdentity(data []byte) (*Identity, error) {
+	return &Identity{}, nil
+}
+
+func ExamplePlugin_main() {
+	p, err := plugin.New("example")
+	if err != nil {
+		log.Fatal(err)
+	}
+	p.HandleRecipient(func(data []byte) (age.Recipient, error) {
+		return NewRecipient(data)
+	})
+	p.HandleIdentity(func(data []byte) (age.Identity, error) {
+		return NewIdentity(data)
+	})
+	os.Exit(p.Main())
+}