Add a special "default" alias

If you set a "default" alias, it will automatically be loaded when you
start a new shell.

README.markdown

@@ -35,3 +35,7 @@ To restore your PATH, you can deactivate it.
 
     nvm deactivate
 
+To set a default Node version to be used in any new shell, use the alias 'default':
+
+    nvm alias default v0.2.5
+

nvm.sh

@@ -206,3 +206,5 @@ nvm()
     ;;
   esac
 }
+
+nvm ls default >/dev/null 2>&1 && nvm use default >/dev/null