Be more strict with the type of `.arclint` properties.

Summary: Although this provides less context in terms of the error message (for example, `Parameter has invalid type. Expected type 'optional regex|list<regex>', got type 'list<string>'.`), I think that it is the right approach. I think that `PhutilTypeSpec::checkMap` should be improved such that additional context is provided in the exception message.

Test Plan: Ran `arc lint`. Modified `.arclint` to contain an invalid regex and ran `arc lint` again.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9089