Add support for Maltese

[tobifjellner]

The PR happened to be submitted before the corresponding issue was created. (Corresponding PR: #761 )
At https://make.wordpress.org/polyglots/2017/06/27/hello-10/ we've received a request to add support for Maltese.

Ethnologue ref: https://www.ethnologue.com/language/mlt

Data:

		$mlt = new GP_Locale(); 
		$mlt->english_name = 'Maltese';
		$mlt->native_name = 'Malti';
		$mlt->lang_code_iso_639_1 = 'mt';
		$mlt->lang_code_iso_639_2 = 'mlt';
		$mlt->lang_code_iso_639_3 = 'mlt';
		$mlt->country_code = 'mt';
		$mlt->facebook_locale = 'mt_MT';
		$mlt->google_code = 'mt';
		$mlt->wp_locale = 'mlt';
		$mlt->slug = 'mlt';
		$mlt->nplurals = 4;
		$mlt->plural_expression = '(n==1 ? 0 : n==0 || ( n%100>1 && n%10010 && n%100<20 ) ? 2 : 3);';