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

maps/registry.go

@@ -2,6 +2,7 @@ package maps
 
 import (
 	"fmt"
+	"sort"
 
 	"github.com/BattlesnakeOfficial/rules"
 )
@@ -21,6 +22,16 @@ func (registry MapRegistry) RegisterMap(id string, m GameMap) {
 	registry[id] = m
 }
 
+// List returns all registered map IDs in alphabetical order
+func (registry MapRegistry) List() []string {
+	var keys []string
+	for k, _ := range registry {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	return keys
+}
+
 // GetMap returns the map associated with the given ID.
 func (registry MapRegistry) GetMap(id string) (GameMap, error) {
 	if m, ok := registry[id]; ok {
@@ -34,6 +45,11 @@ func GetMap(id string) (GameMap, error) {
 	return globalRegistry.GetMap(id)
 }
 
+// List returns a list of maps registered to the global registry.
+func List() []string {
+	return globalRegistry.List()
+}
+
 // RegisterMap adds a map to the global registry.
 func RegisterMap(id string, m GameMap) {
 	globalRegistry.RegisterMap(id, m)