v1.0.0

The stability release, following rasuvaeff/understudy 1.0.0. The plugin's
diagnostic contract is unchanged from 0.8.5; a project on the 0.x bridge
upgrades by changing its engine constraint to ^1.0.

• Requires rasuvaeff/understudy ^1.0, following the family policy: the
  engine is released first and satellites follow it on the same major.