Add ability to display the executor output

[BigRedT]

Fixes #487

Changes proposed in this pull request: Lightly modified version of code pointed by @epwalsh here

Adds the ability to display the executor output. Can be used as follows:

output = executor.execute_step_graph(step_graph)
output.display()

Tested locally and produces an output as follows:

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[BigRedT]

@epwalsh fixed the formatting issues. Not sure if the remaining 3 failing checks are related to this PR?