fix: support Node 12 / Electron 5 (#36)
fix: support Node 12 / Electron 5 (#36)

Replace `v8::Handle` with `v8::Local`
This should not affect behavior in any way since `Handle` was an alias for `Local`.

From `https://v8docs.nodesource.com/node-10.6/d4/da0/v8_8h_source.html#l00342`:
```
  340 // Handle is an alias for Local for historical reasons.
  341 template <class T>
  342 using Handle = Local<T>;
  343 #endif
```

See also:

* https://electronjs.org/blog/nodejs-native-addons-and-electron-5
* https://codereview.chromium.org/1224623004
* https://v8docs.nodesource.com/node-10.6/d2/dc3/namespacev8.html#a569580ff6260b59779fb214d5a1448fd
jduncanator/node-diskusage,jduncanator/node-diskusage,jduncanator/node-diskusage