Allow Pronto::Rubocop instance to be configured with a block

Pronto does not expose the instances of its runners, so there was no way to pass
configuration options to them.  This adds a class method to define a block to be
evaluated in the context of a new Pronto::Rubocop instance after it's
initialized.
Merge pull request #1 from Yesware/allow-instance-configuration

Allow Pronto::Rubocop instance to be configured with a block