Allow hiding 'on_load' attribute.

See #130.
If you remove `hide_on_load`, then `make test` fails with the following:

```
meck_on_load_tests: on_load_test_...*failed*
in function meck_on_load_tests:hide_on_load/0 (test/meck_on_load_tests.erl, line 21)
**error:unexpected_call_to_on_load
```