Add CSV character encoding

[doekenorg]

This PR adds a hook to change the character encoding on a rendered CSV.

add_action( 'gfexcel_renderer_csv_output_encoding', function ( string $encoding ): string {
	return 'ISO-8859-1';
} );

I've use the gfexcel_ prefix to stay consistent instead of adding a new gk/gravityexport/... hook.