gulp performance: emit status updates

[samouri]

summary
I recently started taking advantage of the new gulp performance task. During its execution, it always emits a message about calling yarn install, but then appears to hang until all the measurements are complete. For a single link with 100 runs, this meant 12m without a status update.

This PR adds progress notification to the task. Here is an example of the output:

[rcebulko]

Thanks for doing this! Esther is working on integrating this task into the Travis CI, so while this looks like a step forward I'm going to hold off on approving as she may have plans to implement logging a different way/using amphtml logging conventions.

Since you're using the gulp performance task, it would be super helpful to file any bugs or issues you encounter as issues and tag @ampproject/wg-infra or Esther directly, to help with planning work around it and to get a picture of what could/should be done.

[estherkim]

Thanks @samouri!