*: add --result-format=json format

This is far easier to parse than the default raw format, and provides
the full serialised result structure.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

cmd/gomtree/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"encoding/json"
 	"flag"
 	"fmt"
 	"log"
@@ -30,6 +31,14 @@ var formats = map[string]func(*mtree.Result) string{
 		}
 		return buffer.String()
 	},
+
+	"json": func(r *mtree.Result) string {
+		var buffer bytes.Buffer
+		if err := json.NewEncoder(&buffer).Encode(r); err != nil {
+			panic(err)
+		}
+		return buffer.String()
+	},
 }
 
 func main() {