add the concept of warnings to CPAN::Meta::Validator

[karenetheridge]

this is from a discussion happening at QAH right now, so it will likely be amended later.

CPAN::Meta::Validator currently examines a meta structure and issues a pass/fail decree on it. Authors would benefit by the ability to warn about suspicious things that are still technically valid, but are discouraged or will be ignored by clients.

• existence of 'perl' or core-non-dual-life modules in recommends/suggests prerequisites (these things cannot be upgraded; you get what you get)
• recomended/suggested modules at a lower version than a corresponding entry in 'requires'