Add LXD support

[jiazio]

Support scanning LXD Container .

[kotakanbe]

@jiazio

Thanks for sending P/R.
How do I specify the container type in config.toml?

[jiazio]

Sorry, I forgot to describe how to config. You can specify the container type as follows.

[servers]

[servers.localhost]
host    = "127.0.0.1"
port    = "22"
user    = "ubuntu"
keyPath = "/home/ubuntu/.ssh/id_rsa"
containers = ["${running}"]
[servers.localhost.container]
type    = "lxd"

[kotakanbe]

@jiazio

Thanks!
I tested and confirmed that it works.
I will modify the README at a later date.

I appreciate your contribution.