Changes

add hiddenServers option to !players_by_server component; see #230
The new option allows to remove some server from the list displayed by 
the `!players_by_server` component. Here's an example:
```
- !players_by_server
 playerSet: all_players
 serverHeader:
 - '&b&l>&3 ${server}(${server_player_count}):'
 showServers: ALL
 hiddenServers: [server4, server7]
 playerComponent: "${player name}"
 morePlayersComponent: '... and ${other_count} others'
```
 (commit: aeaccf2)