Add NoChiselNamePrefix to ignore instances in @chiselName (#1383)

Add trait chisel3.experimental.NoChiselNamePrefix which causes
@chiselName to skip naming of the instance effectively preventing it
from prefixing any vals inside the instance. It can be applied to
classes such that all instances of that class have this property, or to
individual instances (via creating an anonymous class inline).

Also add basic ScalaDoc for NoChiselNamePrefix and chiselName.