Do not output notes as JSON as default behaviour

Needing to read the notebook as JSON is not a feature that has been used
often so it shouldn't be the default behaviour. Now that 'zb' will not
try to assume the notebook directory if it doesn't have it configured it
makes sense to output help content by default and move JSON output under
its own subcommand.

cmd/json.go

@@ -0,0 +1,28 @@
+package cmd
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"github.com/msp301/zb/graph"
+	"github.com/spf13/cobra"
+)
+
+var jsonCmd = &cobra.Command{
+	Use:   "json",
+	Short: "Output notes as JSON",
+	Run: func(cmd *cobra.Command, args []string) {
+		book().Notes.Walk(func(vertex graph.Vertex, depth int) bool {
+			jsonStr, err := json.Marshal(vertex)
+			if err != nil {
+				panic(err)
+			}
+			fmt.Println(string(jsonStr))
+			return true
+		}, -1)
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(jsonCmd)
+}

cmd/root.go

@@ -1,15 +1,13 @@
 package cmd
 
 import (
-	"encoding/json"
 	"fmt"
 	"log"
 	"os"
 	"path"
 
 	"github.com/msp301/zb/config"
 	"github.com/msp301/zb/editor"
-	"github.com/msp301/zb/graph"
 	"github.com/spf13/cobra"
 	"github.com/spf13/viper"
 )
@@ -21,14 +19,7 @@ var rootCmd = &cobra.Command{
 	Use:   "zb",
 	Short: "A Zettelkasten notebook utility",
 	Run: func(cmd *cobra.Command, args []string) {
-		book().Notes.Walk(func(vertex graph.Vertex, depth int) bool {
-			jsonStr, err := json.Marshal(vertex)
-			if err != nil {
-				panic(err)
-			}
-			fmt.Println(string(jsonStr))
-			return true
-		}, -1)
+		cmd.Help()
 	},
 }