Adopt Cedar's SPEC_BEGIN/SPEC_END construct

Optionally allow SPEC_BEGIN and SPEC_END to be used instead of the go_bandit() construct.

I'm borrowing this from the [Cedar](https://github.com/pivotal/cedar) project. I feel it helps make the test syntax more generic because it doesn't require the tests to include the name of the test framework. Granted, the `using` statements still have the framework name, but they can always be hidden in the helper header.