Add invitation survey

```sh
$ ./bin/flyctl orgs invite
? Select organization: paypack (paypack)
? User email: name@example.com
Org                  Email                Redeemed
----                 ----                 ----
paypack              name@example.com     false
```

cmd/input.go

@@ -376,3 +376,14 @@ func isIntPort(val interface{}) error {
 
 	return errors.New("couldn't convert to string")
 }
+
+func inputUserEmail() (email string, err error) {
+	prompt := &survey.Input{
+		Message: "User email:",
+	}
+	if err := survey.AskOne(prompt, &email); err != nil {
+		return email, err
+	}
+
+	return email, nil
+}

cmd/orgs.go

@@ -140,17 +140,33 @@ func runOrgsShow(ctx *cmdctx.CmdContext) error {
 }
 
 func runOrgsInvite(ctx *cmdctx.CmdContext) error {
-	orgslug := ctx.Args[0]
+	var orgSlug, userEmail string
 
-	org, err := ctx.Client.API().GetOrganizationBySlug(orgslug)
+	switch len(ctx.Args) {
+	case 0:
+		org, err := selectOrganization(ctx.Client.API(), "")
+		if err != nil {
+			return err
+		}
+		orgSlug = org.Slug
 
+		userEmail, err = inputUserEmail()
+		if err != nil {
+			return err
+		}
+	case 1:
+		// TODO: Validity check on org
+		orgSlug = ctx.Args[0]
+	case 2:
+		userEmail = ctx.Args[1]
+	}
+
+	org, err := ctx.Client.API().GetOrganizationBySlug(orgSlug)
 	if err != nil {
 		return err
 	}
 
-	email := ctx.Args[1]
-
-	out, err := ctx.Client.API().CreateOrganizationInvite(org.ID, email)
+	out, err := ctx.Client.API().CreateOrganizationInvite(org.ID, userEmail)
 	if err != nil {
 		return err
 	}