disable ANSI escape codes if not tty

[connorjclark]

Would be nice if running pwmetrics in a non-tty environment suppressed ANSI code output.

Otherwise, output looks like:

Launching Chrome
  [32m✓[0m Success: [32mRun 1 of 3 finished successfully.[0m
Launching Chrome
  [32m✓[0m Success: [32mRun 2 of 3 finished successfully.[0m
Launching Chrome
  [32m✓[0m Success: [32mRun 3 of 3 finished successfully.[0m
        ☆  Median run  ☆

[denar90]

Hope migrating to https://github.com/chalk/chalk can solve this problem.