Provide a convenience method to launch treadle REPL on a chisel design

[chick]

firrtl-interpreter had a this implemented in chisel-iotesters.
Consider adding to chisel3 if it retains it's dependency on treadle
otherwise this should probably go into chiseltest.

Stretch goal
Make an annotation/cli-flag that would launch the repl on a test failure
with rollback enabled this could allow better diagnostics