Added ingore file, to stop node_modules from being added
Running app.js kept giving me a node error.
I added the missing dependancies in the package.json file
(these can then be installed using `npm install`).
Added path dependancy for file extension check
More robut argument checks & position variety

- The app now checks if enough arguments have been given.

- If the first argument given is not a number (i.e. not a frame
  number), the frameStart and frameEnd variables are set by the
  files in the given folder.

- The image file type extension is no longer hard coded as `.png`.
  This was because ffmpeg can also output jpgs.