Run plugin only during test runs, and initialize only once

[crisptrutski]

Think the best way to spare unnecessary is to use the add-hook to intercept midje tasks, and skip adding middleware.

Additionally when running lein midje :autowatch from Instar, I noticed that the existing middleware function is actually firing three times. This is bad enough when just running eval-in-project and doing IO, but when multiple filesystem watchers are being created the cost becomes more noticeable.

Should be able to wrap the initialization for file watcher in a "execute at most once" to avoid this sort of issue occurring again (although fairly benign - just inefficient)