[File inclusion] add support for representing a text file as an array of lines

[nemesifier]

The only way to add strings containing new lines in JSON is to use the escaped form \n.

Therefore to add a file with multiple lines in netjsonconfig now one has to do:

{
    "files": [
        {
            "path": "/root/.ssh/authorized_keys",
            "contents": "sshkey1\nsshkey2\nsshkey3"
        }
    ]
}

It would be nice to add support to an array representation, each element of the array representing a line of a file:

{
    "files": [
        {
            "path": "/root/.ssh/authorized_keys",
            "contents": [
                "sshkey1",
                "sshkey2",
                "sshkey3"
            ]
        }
    ]
}

This would also improve readability a great deal.