Nicer game invites

[wvffle]

Pretty much continuation of #6

Currently, the message you get when someone invites you is:

[lobbyinvite ="" appid="730" lobbyid="109775241107229798"]

I'd love it to be:

{{username}} invited you to play {{app name}}:
<a href="steam://joinlobby/{{app id}}/{{lobby id}}/{{inviter profile id}}">Join lobby</a>

If there are clients that wouldn't open the steam:// url then instead of sending Join lobby simply send the same as href attr