watch files for changes

[andrewharvey]

mbview could watch for changes the source mbtiles and reload the server and refresh the client when changes occur.

 var watcher = chokidar.watch(file, {
   persistent: false,
   usePolling: true, 
   awaitWriteFinish: true
 });
 
 watcher.on('change', function(path) {
   console.log(path + ' changed');
 });