Minitest support

Not finished yet¹, but a PR to gauge whether you would like to have minitest support in the main Bogus repo (via an optional `require 'bogus/minitest'`, akin to RSpec support), and whether I get the wirings about right.

¹ missing: docs/Cucumber features, `verify_contract` support, hooking mock checks to `MiniTest::Unit` rather than `MiniTest::Spec`, splitting into proper file hierarchy, etc.