Proposal: add ability to silence exception instances

[johnnyshields]

I have some cases where I do:

rescue MyException => e
  Bugsnag.notify(e, {meta: data})
  raise e
end

The exception is raised twice in Bugsnag. I'd prefer if I could do something like:

rescue MyException => e
  Bugsnag.notify(e, {meta: data})
  e.bugsnag_silence!
  raise e
end

And that instance wouldn't generate a Bugsnag when it hit's Bugsnag's global catching logic.