Expose lush global as a standard library (issue #26)

Register luaopen_lush() in stdlibs so the lush table is available as a
global. Users can now extend the shell by adding functions to
lush.builtins. The OP_LUSH VM access via integer keys is preserved.

lcmd.h

@@ -23,4 +23,6 @@ int lushCmd_getenv (lua_State *L);
 int lushCmd_setenv (lua_State *L);
 int lushCmd_subcmd (lua_State *L);
 
+LUAMOD_API int luaopen_lush (lua_State *L);
+
 #endif