Added beforeReload hook

[lucafabbian]

Add beforeReload option.
Now you can run a javascript function every time a change is detected.

Example:

liveServer.start({
  beforeReload: (changePath) => { 
    console.log(changePath + " file has changed! Reloading...")
  } 
});

It supports Promises and async functions too:

liveServer.start({
  beforeReload: async(changePath) => { 
    /* await some async resource
      the page will not reload until the resource is ready */
  } 
});