fix injection of cth_readable to allow for hook configuration

[TheGeorge]

With the current behavior, rebar3 removes all cth_readable hooks from the config and adds the ones it wants. This however only works when using atoms only (which is normally used).

If we add a config to the hooks in the rebar.confg, the whole thing breaks:

{cth_hooks, [{cth_readable_failonly, []}]}

because (Hooks -- AllReadableHooks) assumes everything to be atoms. The type signature however is:

{ct_hooks,[my_cth_module]}
{ct_hooks,[{my_cth_module,[{debug,true}]}]}
{ct_hooks,[{my_cth_module,[{debug,true}],500}]}

(see https://erlang.org/doc/apps/common_test/ct_hooks_chapter.html)

This change is needed for ferd/cth_readable#34 to work correctly.