Add the label command

This command can currently only list the labels associated with a git
repository. In the future, it will also allow the creation of new
labels and modifiaction of existing labels.

commands/label.go

+package commands
+
+import (
+	"github.com/github/hub/github"
+	"github.com/github/hub/ui"
+	"github.com/github/hub/utils"
+)
+
+
+var (
+	cmdLabel = &Command{
+		Run: listLabels,
+		Usage: `
+label
+`,
+		Long: `Manage GitHub labels for the current repository.
+
+## Commands:
+
+With no arguments, show a list of open issues.
+`,
+	}
+)
+
+func init() {
+	CmdRunner.Use(cmdLabel)
+}
+
+func listLabels(cmd *Command, args *Args) {
+	localRepo, err := github.LocalRepo()
+	utils.Check(err)
+
+	project, err := localRepo.MainProject()
+	utils.Check(err)
+
+	gh := github.NewClient(project.Host)
+
+    if args.Noop {
+		ui.Printf("Would request list of labels for %s\n", project)
+	} else {
+		labels, err := gh.FetchLabels(project)
+		utils.Check(err)
+
+		for _, label := range labels {
+			ui.Printf(formatLabel(label))
+		}
+	}
+
+	args.NoForward()
+}
+
+func formatLabel(label github.IssueLabel) string {
+    format := "%l%n"
+
+	placeholders := map[string]string{
+		"l":  label.Name,
+	}
+
+	return ui.Expand(format, placeholders, false)
+}