Add support for datasets

[jonatanklosko]

Closes #16.

@dustinfarris if you have a use case for this feature, please test this branch and let me know if it gets you where you need :)

{:vega_lite, "~> 0.1.1", github: "livebook-dev/vega_lite", branch: "jk-datasets", override: true}

Example usage alternative to data_from_values:

Vl.new(width: 400, height: 400)
|> Vl.datasets_from_values(
  mydata: [
    %{iteration: 1, score: 1},
    %{iteration: 10, score: 10},
    %{iteration: 100, score: 100}
  ]
)
|> Vl.data(name: "mydata")
|> Vl.mark(:line)
|> Vl.encode_field(:x, "iteration", type: :quantitative)
|> Vl.encode_field(:y, "score", type: :quantitative)

[dustinfarris]

It works!