docs: add an example Go plugin

docs/example-go-plugin/plugin.go

@@ -0,0 +1,39 @@
+// Package exampleplugin is an example Go plugin for koushin.
+//
+// To enable it, import this package from cmd/koushin/main.go.
+package exampleplugin
+
+import (
+	"fmt"
+	"net/http"
+
+	"git.sr.ht/~emersion/koushin"
+	koushinbase "git.sr.ht/~emersion/koushin/plugins/base"
+)
+
+func init() {
+	p := koushin.GoPlugin{Name: "example"}
+
+	// Setup a function called when the mailbox view is rendered
+	p.Inject("mailbox.html", func(ctx *koushin.Context, kdata koushin.RenderData) error {
+		data := kdata.(*koushinbase.MailboxRenderData)
+		fmt.Println("The mailbox view for " + data.Mailbox.Name + " is being rendered")
+		// Set extra data that can be accessed from the mailbox.html template
+		data.Extra["Example"] = "Hi from Go"
+		return nil
+	})
+
+	// Wire up a new route
+	p.GET("/example", func(ctx *koushin.Context) error {
+		return ctx.String(http.StatusOK, "This is an example page.")
+	})
+
+	// Register a helper function that can be called from templates
+	p.TemplateFuncs(map[string]interface{}{
+		"example_and": func(a, b string) string {
+			return a + " and " + b
+		},
+	})
+
+	koushin.RegisterPluginLoader(p.Loader())
+}