add 'map' cli command to provide map information (#100)

* add 'map' cli command

- provides the following map information functions:
  - list all available maps in the global registry
  - display map metadata

- update docs with map command examples

* add list and info subcommands to map cli command

* rename map command list and info factory functions

* add --all flag to map info subcommand

* handle cmd.Help error

cli/commands/list.go

@@ -0,0 +1,22 @@
+package commands
+
+import (
+	"fmt"
+
+	"github.com/BattlesnakeOfficial/rules/maps"
+	"github.com/spf13/cobra"
+)
+
+func NewMapListCommand() *cobra.Command {
+	var listCmd = &cobra.Command{
+		Use:   "list",
+		Short: "List available game maps",
+		Long:  "List available game maps",
+		Run: func(cmd *cobra.Command, args []string) {
+			for _, m := range maps.List() {
+				fmt.Println(m)
+			}
+		},
+	}
+	return listCmd
+}