Allow DSL methods to be checked for length

The MethodLength cop hasn't been able to check DSL-style methods in
RSpec, Sinatra, and others. Especially in the case of Sinatra apps, the
content in the DSL is the most important part of the app, so it's
important that this is checkable.

This change adds support for a configurable list of DSL methods which
are treated as methods when checking method length. To be checked, the
methods must be invoked with a block and no receiver, and then the
contents of the block are considered the "body" of the method.