Allow dash characters in subdomain.

Thank you @zackslash via https://github.com/localtunnel/server/pull/47

Rebased to latest master.

server.js

@@ -81,7 +81,7 @@ module.exports = function(opt) {
         const req_id = parts[1];
 
         // limit requested hostnames to 63 characters
-        if (! /^[a-z0-9]{4,63}$/.test(req_id)) {
+        if (! /^(?:[a-z0-9][a-z0-9\-]{4,63}[a-z0-9]|[a-z0-9]{4,63})$/.test(req_id)) {
             const msg = 'Invalid subdomain. Subdomains must be lowercase and between 4 and 63 alphanumeric characters.';
             ctx.status = 403;
             ctx.body = {