Make node sniffer more selective

[clintongormley]

Add support for:

sniff_node_types: {
    data:   "yes | no | fallback",
    master: "yes | no | fallback",
    client: "yes | no | fallback"
}

prefer_nodes: "192.168.*|rack:one|etc"

Closes #71

[darxriggs]

This was closed without a comment. Does this mean it will not be implemented?

At least the default implementation of should_accept_node still accepts every node in v7.715.