Plugin tries to process non Shortcode files (custom shortcodes)

## Problem
Recently, I've had an error that the Shortcode plugin somehow tried to process a `.gitkeep` file in the custom shortcodes folder specified with `custom_shortcodes`.

## How I noticed the problem
I don't know why and how, but my `.gitkeep` file had two empty lines instead of just one by accident, and that caused a single empty line at the top of every page. After I removed the new line or disabled the plugin, the problem was gone.

## Security
This issue could be a bigger security risk, since the plugin parses files which are not meant to be a shortcode.

## Possible solution
Since all shortcodes are built like this:,
```php
class CustomShortcode extends Shortcode {
public function init() {
$this->shortcode->getHandlers()->add('custom', function (ShortcodeInterface $sc) {
return 'some custom content';
});
}
}
```

I think it must be possible to only "parse" shortcode files. Or even only executing PHP code which is a class and extends `Shortcode`.

---

I'm not very familiar with PHP yet, so this could be a wrong assumption, but why was the plugin built this way? Isn't it easier to just run child classes of `Shortcode`