Add exclude option

Allow specific fields to be excluded. By default it will exclude "file" & "hidden" inputs.
At the moment, trying to persist a "file" input will throw a JavaScript error when trying to set its value to the stored vale:
```js
Uncaught InvalidStateError: Failed to set the 'value' property on 'HTMLInputElement': This input element accepts a filename, which may only be programmatically set to the empty string. 
```

See Issue #30