Add Command

The `add` command adds a dependency to the dub.json/dub.sdl file.

Running `dub add vibe-d` is equivalent to manually adding the following line to dub.sdl (X.Y.Z represents the latest version):
dependency "vibe-d" version="~>X.Y.Z"

Or the following in dub.json:
"dependencies": {
    "vibe-d": "~>X.Y.Z"
}