Support for statfs() and other newer fs functions in more recent versions of node.js

[Eric24]

Are newer fs functions "automatically" included in the promisified list of functions, or must these functions specifically be listed in the fs-extra source?

[RyanZim]

Need to be listed in https://github.com/jprichardson/node-fs-extra/blob/master/lib/fs/index.js; PR welcome to add them.