add option 'keys' to force ordering

[r3mi]

currently there is no way to force that lines are printed in a given order.
Specifically, Object.keys does not guarantee that keys are returned in insertion order (for example, on chrome and node, keys with numeric names e.g. '0' are returned first and in sort order).
This patch add a 'keys' option to force ordering

var data = {
  ferrets: 20,
  cats: 12,
  dogs: 30,
  koalas: 3
};
console.log(bars(data, { keys: ['cats', 'dogs', 'ferrets'] }));

[r3mi]

PS : should I push this patch here, or on https://github.com/ben-ng/bars from @ben-ng , which is the source of the NPM package ?