lifecycle 0.2.0

• Lifecycle warnings are now displayed once every 8 hours.

• Added experimental signal_experimental() and signal_superseded()
  functions.

• Added the "superseded" lifecycle stage to the documentation.

• deprecate_stop() now mentions that function is defunct (#28).

• New expect_deprecated() and expect_defunct() functions for
  testting lifecycle warnings and errors. expect_deprecated()
  automatically sets the lifecycle_verbosity option to "warning"
  to enforce warnings at each invokation rather than once per session.

• New syntax "foo(arg = 'can\\'t be a baz')" to describe that specific inputs
  for an argument are deprecated (#30, @krlmlr).

• New is_present() function to test whether the caller has supplied a
  deprecated() function.