Use native Object.assign instead of xtend

[jimmywarting]

xtend can easily be replaced with Object.assign

This reduce the dependencies and possible also avoid duplicated versions where some package don't use the same version range for the xtend package

// immutable
Object.assign({}, a, b)

// mutable
Object.assign(a, b)

[mattdesl]

This would lead to a breaking version change in budo, which is fine but I may as well roll other features into the same breaking change. I don't see how removing a single dependency will have any noticeable impact, so I am not in any real rush to change this.