Add (truncated) preview to snippets command

[sebkuip]

This update changes the ?snippet command. When the snippet command is invoked with no snippet name, but snippets do exist, it will not only show a list of all snippets, but also show a (possibly truncated to a length of 350 characters) preview. Also changed the amount of snippets per page from 15 to 10 per page to accommodate the more space taken up.