Fix username/mention regexes

* Allow underscore instead of hypen
* Fix domain part handling
* Add tests for remote mention

src/text/parse/elements/mention.ts

@@ -4,7 +4,7 @@
 import parseAcct from '../../../acct/parse';
 
 module.exports = text => {
-	const match = text.match(/^(?:@[a-zA-Z0-9\-]+){1,2}/);
+	const match = text.match(/^@[a-z0-9_]+(?:@[a-z0-9\.\-]+[a-z0-9])?/i);
 	if (!match) return null;
 	const mention = match[0];
 	const { username, host } = parseAcct(mention.substr(1));