Use N-API

[mischnic]

I did this mainly because the current master doesn't compile on Node 11 (the deprecation warnings in Node 10 turned into fatal errors).

Uses https://github.com/nodejs/node-addon-api

The code might not be optimal yet: Interesting/Strangely, this changes the write performance (NAN usually 0.7-1.2 ms and NAPI 1.5-2.4 or even 4 ms on macOS):
NAN:

read: 12.758ms
[ { path:
     'fschanges/token.txt',
    type: 'update' } ]
write: 0.747ms

N-API:

node index.js
read: 11.063ms
[ { path:
     'fschanges/token.txt',
    type: 'update' } ]
write: 1.256ms