Expose custom safe_repr config

[sl0thentr0py]

@sl0thentr0py Thanks, yes, I think we prefer option 2 - the option for a custom repr function to be tried in safe_repr (my current understanding of how everything works is that this will both solve our needs and reduce the likelihood of this bug to creeping back in).

Thanks for giving us the choice 🙂

Originally posted by @philipstarkey in #3409 (comment)

[sl0thentr0py]

@philipstarkey merged, will be in next release