Require authentication for post operations in wf

Unlike Write.as, WriteFreely doesn't support anonymous (unauthenticated)
publishing. With these changes, we reflect that in wf-cli by wrapping
creating, updating, and deleting posts in a func that checks
authentication state.

Ref T586

cmd/wf/commands.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/writeas/writeas-cli/config"
+	"github.com/writeas/writeas-cli/executable"
+	cli "gopkg.in/urfave/cli.v1"
+)
+
+func requireAuth(f cli.ActionFunc, action string) cli.ActionFunc {
+	return func(c *cli.Context) error {
+		u, err := config.LoadUser(c)
+		if err != nil {
+			return cli.NewExitError(fmt.Sprintf("couldn't load config: %v", err), 1)
+		}
+		if u == nil {
+			return cli.NewExitError("You must be authenticated to "+action+".\nLog in first with: "+executable.Name()+" auth <username>", 1)
+		}
+
+		return f(c)
+	}
+}

cmd/wf/main.go

@@ -34,13 +34,13 @@ func main() {
 	app.ExtraInfo = func() map[string]string {
 		return appInfo
 	}
-	app.Action = commands.CmdPost
+	app.Action = requireAuth(commands.CmdPost, "publish")
 	app.Flags = append(config.PostFlags, flags...)
 	app.Commands = []cli.Command{
 		{
 			Name:   "post",
 			Usage:  "Alias for default action: create post from stdin",
-			Action: commands.CmdPost,
+			Action: requireAuth(commands.CmdPost, "publish"),
 			Flags:  config.PostFlags,
 			Description: `Create a new post on WriteFreely from stdin.
 
@@ -67,19 +67,19 @@ func main() {
    
    If posting fails for any reason, 'wf' will show you the temporary file
    location and how to pipe it to 'wf' to retry.`,
-			Action: commands.CmdNew,
+			Action: requireAuth(commands.CmdNew, "publish"),
 			Flags:  config.PostFlags,
 		},
 		{
 			Name:   "publish",
 			Usage:  "Publish a file",
-			Action: commands.CmdPublish,
+			Action: requireAuth(commands.CmdPublish, "publish"),
 			Flags:  config.PostFlags,
 		},
 		{
 			Name:   "delete",
 			Usage:  "Delete a post",
-			Action: commands.CmdDelete,
+			Action: requireAuth(commands.CmdDelete, "delete"),
 			Flags: []cli.Flag{
 				cli.BoolFlag{
 					Name:  "tor, t",
@@ -99,7 +99,7 @@ func main() {
 		{
 			Name:   "update",
 			Usage:  "Update (overwrite) a post",
-			Action: commands.CmdUpdate,
+			Action: requireAuth(commands.CmdUpdate, "update"),
 			Flags: []cli.Flag{
 				cli.BoolFlag{
 					Name:  "tor, t",