Validate config values for arrays and hashes

[Morriar]

Motivation

Since we write Tapioca config as a YAML file, it's easy to make type errors that will result in unexpected behaviors.

For example, even if this config file looks ok, the msgpack gem will not be turned typed: false:

gem:
  typed_overrides:
    msgpack: false

Because we expect the values inside the typed_overrides hash to be strings rather than booleans.

This actually happened today to one of our users: #895 (comment). Having an error message earlier would have saved both of us some time and confusion.

Implementation

The solution choose here seems a bit hack-ish but we can't rely on Thor expected types for the content of :array and :hash options:

Once we validated all others criteria for options keys and values we add an additional check for arrays and hashes to make sure the values (and keys in the case of a hash) are all of the type String.

Even if rather limited, this works in practice because the only arrays and hashes we take in the config always expect string (iirc?).

I guess we can revisit this later if we add different options but I'm open to better suggestions.

Tests

See automated tests.

[paracycle]

Nice one, thanks!