Add an environment variable to detect plugin usage in the telemetry

[jubianchi]

The telemetry report looks for some environment variables to determine on where we are running atoum from.

It would be cool if the plugin could add its own variables (for example ATOM=true) when we run atoum through it so we can detect this and graph plugin usage.

[Grummfy]

seems it's around lib/runner.coffee that it must be added, but not sure the good way to do it. Adding ATOM=true in front of the command ?
will it work on windows?