More robust argument checks & position variety 

- App.js 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 `.jpg`s.

- I added the missing dependencies in the `package.json` file
(these can then be installed using `npm install`).

- I added a `.gitingore` file to stop `node_modules` from being added/commited