[plugin] add simple collect() profiling

Similar to commit fd68a0c; add support for outputing messages
indicating the time taken to call collect() for each plugin:

[...]
  [plugin:systemtap] collected plugin 'systemtap' in 16.8759040833
  [plugin:systemd] collected plugin 'systemd' in 19.5138418674
  [plugin:block] collected plugin 'block' in 40.066119194
  [plugin:yum] collected plugin 'yum' in 101.271748066

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>