fix(deps): update node and electron (#36)

Goals: improve security, fix #22.

Also fixes crash when quitting the app on macOS.

Changes:
- upgrade from node version 8 to 12
- upgrade from electron version 1 to 9 + latest version of associated dependencies
- move most dependencies to devDependencies (see electron/packager#827)
- only build x64 binaries (see https://www.electronjs.org/blog/linux-32bit-support)
- only build for darwin/macos, windows and linux
- re-format js files using latest version of standard

.nvmrc

@@ -1 +1 @@
-v8.16.2
+v12.18.2