add execPath and nodeOptions in service config

[artmen1516]

This PR is to add a similar functionality as in node-windows for execPath and nodeOptions,
As a developer, I should be able to configure the executable that will launch a Service with Custom Args (not only --harmony)

This is useful in a packaged application since the electron executable by default is all the app itself, for example /Applications/MyApp.app/Contents/MacOS/MyApp, if this executable tries to install the script will run all the application.
So what we are doing is to include the Electron binary or Node Binary, and use it in execPath in node-windows, that works well.
The problem is when we tried to use the same for node-mac.

Reading the contribution guidelines...
Contributions must be cross-platform compatible... node-windows/mac/linux share a common API.

This change will help to close the gap between the available configuration options in windows and mac.

Tested locally in my mac, with this change the plist passes the correct execPath provided in the config.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
  <dict>
    <key>Title</key>
    <string>myappservice</string>
    <key>Label</key>
    <string>myappservice</string>
    <key>ProgramArguments</key>
    <array>
      <string>/Applications/MyApp.app/Contents/Resources/electron/dist/Electron.app/Contents/MacOS/Electron</string>