standardize stats file

[ranyitz]

Emitting a stats file can be an awesome feature for people that want to :

1. Audit their build performance (as we keep performance in mind)
2. Give an estimation for how long your build will take on its next run (as we keep user experience in mind).

Haste-plugin-stats

Saves statistics of your build history

why ?

Other plugins can later use this file to create various awesome features.

1. Time estimation based on last builds.
2. Build monitoring.
3. Investigate why your build time drastically changed.
4. Find frequent errors.

and more...

The problems

If we want this plugin to have a significant effect, other plugins should rely on its existence.
If we'll let users change its location via options, other plugins wouldn't know.