Upgraded chalk dependency (thank you @yunzhu-li!)

Node API

You can now use tap-difflet inside node code:

var tapDifflet = require('tap-difflet');

var formatter = tapDifflet({
  pessimistic: true // Only output failed tests. `false` by default.
});

Thanks to @infernalmaster for separating TAP processing from CLI code. ❤️

Slightly cleaner output. See #7

Added pessimistic option (use -p or --pessimistic on the command line) -- only failures are printed.