feat: add support for Arrays

[niftylettuce]

Right now you can't pass an Array to sortKeys(...) - ideally we should be able to pass an Array (e.g. an Array of objects) or a plain Object to the method exported.

Related: #10

[sindresorhus]

Why? Do you have any actual use-cases for this? This module is about sorting the keys of objects. Feels weird to support array as an input, even if it's an array of objects.

[coreyfarrell]

If you know you're dealing with an array of objects you can do the following:

const sorted = arrayOfObjects.map(item => sortKeys(item, {deep: true}));

[jamestalmage]

This would improve the experience for hash-obj, which breaks on arrays because of this module.

This would be a useful addition for any number of libraries that need a consistent scan order of an arbitrary object (i.e. I don't know if the user is giving me an array, or a plain obj).

Additionally, it is not trivial to wrap this for Array's as @coreyfarrell suggests, as you would miss out on circular reference when deep-sorting