feat(plugins): add rudimentary plugin system

examples/plugins/example/main.go

@@ -0,0 +1,52 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package main
+
+import (
+	"code.vikunja.io/api/pkg/events"
+	"code.vikunja.io/api/pkg/log"
+	"code.vikunja.io/api/pkg/models"
+	"code.vikunja.io/api/pkg/plugins"
+
+	"github.com/ThreeDotsLabs/watermill/message"
+)
+
+type ExamplePlugin struct{}
+
+func (p *ExamplePlugin) Name() string    { return "example" }
+func (p *ExamplePlugin) Version() string { return "1.0.0" }
+func (p *ExamplePlugin) Init() error {
+	log.Infof("example plugin initialized")
+
+	events.RegisterListener((&models.TaskCreatedEvent{}).Name(), &TestListener{})
+
+	return nil
+}
+func (p *ExamplePlugin) Shutdown() error { return nil }
+
+func NewPlugin() plugins.Plugin { return &ExamplePlugin{} }
+
+type TestListener struct{}
+
+func (t *TestListener) Handle(msg *message.Message) error {
+	log.Infof("TestListener received message: %s", string(msg.Payload))
+	return nil
+}
+
+func (t *TestListener) Name() string {
+	return "TestListener"
+}