Add customizable greeting to index page

Allow an organization to copy the greeting.html file out of the themes
directory, customize it, then point the greeting.file config option at
it so that it will survive Inbucket upgrades.

web/root_controller.go

@@ -3,12 +3,20 @@ package web
 import (
 	"fmt"
 	"github.com/jhillyerd/inbucket/config"
+	"html/template"
+	"io/ioutil"
 	"net/http"
 )
 
 func RootIndex(w http.ResponseWriter, req *http.Request, ctx *Context) (err error) {
+	greeting, err := ioutil.ReadFile(config.GetWebConfig().GreetingFile)
+	if err != nil {
+		return fmt.Errorf("Failed to load greeting: %v", err)
+	}
+		
 	return RenderTemplate("root/index.html", w, map[string]interface{}{
 		"ctx": ctx,
+		"greeting": template.HTML(string(greeting)),
 	})
 }