Add promise support for fs.rm()

[RyanZim]

Fixes #841

[ChALkeR]

utACK.

When I attempted to test, other local tests fail on master, investigating.
Upd: with #861, tests pass.

Side note: not sure why this code needs universalify dep at all, since this already has Node.js 10 in engines, and Node.js >=8 has util.promisify support. Will check separately. Upd: has both promise and callback APIs.

[ChALkeR]

Q: why not just export fs.promises.rm?

There are no versions of Node.js where fs.rm is available and fs.promises.rm isn't available.

Refs:

• https://nodejs.org/api/fs.html#fs_fs_rm_path_options_callback
• https://nodejs.org/api/fs.html#fs_fspromises_rm_path_options

Upd: the answer is "to be compatible with both promises and callbacks".