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.

lualib.h

@@ -54,6 +54,10 @@ LUAMOD_API int (luaopen_table) (lua_State *L);
 #define LUA_UTF8LIBK	(LUA_TABLIBK << 1)
 LUAMOD_API int (luaopen_utf8) (lua_State *L);
 
+#define LUA_LUSHLIBNAME	"lush"
+#define LUA_LUSHLIBK	(LUA_UTF8LIBK << 1)
+LUAMOD_API int (luaopen_lush) (lua_State *L);
+
 
 /* open selected libraries */
 LUALIB_API void (luaL_openselectedlibs) (lua_State *L, int load, int preload);