Validate rule configs in `elvis.config`

[bormilan]

Now Elvis can spot if a non-existent rule was given in elvis.config, but not a bad configuration.
Non-existent or wrong(an int for a string var) configurations should be spotted, and a warning would be helpful for the user.

[paulo-ferraz-oliveira]

I believe we're almost entering the domain of DSL, so maybe coding all the validations in pure Erlang might not be the best approach (certainly possible) for "ease of maintenance" reasons, but we should find a solution where we can express the expectations almost like a typespec.

[bormilan]

You are right, maybe at first the name of the keys is enough. The type checks may be a little too ambitious.

[paulo-ferraz-oliveira]

I didn't mean to contradict your suggestion (maybe that came out wrong), quite the opposite; I believe that with some time we could extend the validation and such to something complementing pure Erlang... I like validations 😄 Type checks should be Ok, since if they don't throw in the configuration parsing they'll throw later.

[paulo-ferraz-oliveira]

I've toyed with the types idea when implementing #515, but ultimately went with "validate on keys", for the time being. Types are very ambitious, especially given some are lists that'll change with time and we haven't expressed the defaults in a way that could mean easily identify them. I've now considered this as closable in #515 given the relevant considerations are implemented.