Add spec for throwing interdependent plugins

When plug-ins depend on each other in `attach` functions,
interdependent relationships (`A` depends on `B`, `B` depends
on `A`), throws a `RangeError`.

Issue: https://github.com/wooorm/retext/issues/20
Fix throwing interdependent plugins

Closes GH-20: Interdependent relationships, plug-ins depending
on each other in `attach` functions (`A` depends on `B`, `B`
depends on `A`) no longer throw `RangeError`s.

Issue: https://github.com/wooorm/retext/issues/20