Add missing fuse option: nonempty

[jetibest]

The nonempty fuse option is missing:

nonempty
    Allows mounts over a non-empty file or directory. By default these
    mounts are rejected to prevent accidental covering up of data, which
    could for example prevent automatic backup.

I added it myself at line number 195 of index.js in _fuseOptions():

    if (this.opts.nonempty) options.push('nonempty')