Add child-theme support

[heavybeard]

Great npm plugin, but it miss the child-theme support.
Have you already thought about it?

The name of the parent-theme can be entered here

"dependencies": {
    "parent-theme": "Parent Theme Name"
}

Let me know :)

[crispossa]

I added some others missing keys, such as License and Textdomain and submitted a Pull Request: #5