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)

[joehand]

Yes!

[joehand]

Okay, getting closer here. The leftover dependencies that use xtend are:

• dat-json
• dat-registry
• subcommand
• dat-node

And then Dat itself =)