Add alternative API that doesn't require custom it

[jappeace]

At riskbook we have a fairly large test suite, adding a custom it to that test suite isn't done easily. This alternative 'testThese' method should allow you to run hspec slow on entire test suites without any customization.
These changes don't impact the existing API at all.

An example of the output is as follows:

Slow examples:
2.072315639s: ../riskbook-io-test/src/Handler/AdminSpec.hs[62:5]
        updates company and saves its organisation
2.294750009s: ../riskbook-io-test/src/Handler/PackSpec.hs[159:3]
        cedent can share a risk pack to a broker

The descriptions come from the test items themselves, as do the source locations.