Use ES2015 Proxies

[phated]

I wanted to open this issue as a place to track things we could use Proxies to implement.

• Currently, unsetting base to fallback to cwd requires setting base to null or undefined. With Proxies, I believe we could watch for delete file.base and unset it.
• We could probably do some cool stat stuff with Proxies. Not sure what though.

More information about Proxies at https://github.com/mikaelbr/proxy-fun

[phated]

In the docs, we currently tell people to treat history and it's properties as read-only. I think we could enforce this with proxies.

[darsain]

Yeah, that would be a great use for a proxy here.