export declared types from getopt.erl

This patch exports the externally useful declared types so clients
may make use of them in thier codebase.

src/getopt.erl

@@ -13,6 +13,13 @@
 
 -export([parse/2, usage/2, usage/3, usage/4]).
 
+-export_type([arg_type/0,
+	      arg_value/0,
+	      arg_spec/0,
+	      simple_option/0,
+	      compound_option/0,
+	      option/0,
+	      option_spec/0]).
 
 -define(TAB_LENGTH, 8).
 %% Indentation of the help messages in number of tabs.