Add option to ignore non-exported functions on max_function_arity

[elbrujohalcon]

Is your feature request related to a problem? Please describe

Private functions having too many arguments are not as much of a problem as exported ones. They should not be judged too severely.

Describe the solution you'd like

I would add a parameter like…

• non_exported_max_arity :: pos_integer()

…with a very very large default value (e.g. 20)

Describe alternatives you've considered

An alternative would be to just ignore non-exported functions or have a flag like apply_to :: exported | non_exported | both that defaults to exported. But I think my proposal above is more flexible.

[paulo-ferraz-oliveira]

I'd simply ignore the private functions (but yeah, going with a super-high default might not hurt, either). Once they're exposed the rule kicks in and changes might be in order.