Support additional job metadata

[richardartoul]

@spence 's original comment:

What Andrew said about tagging resonated. It would be super nice to know (from the mobile app) some broad information about my job. e.g.,

• Platform: Keras/Tensorflow
• Commit SHA: JQL90I3
• Data source: cats_vs_dogs_20161101.csv
• etc.

We could represent this like

hd_options = {
  "source": "Keras/Tensorflow",
  "commit": ...
}
# ...
@monitor("dogs vs. cats", hd_options)

[richardartoul]

What are the exact metadata fields we want to support? Do you want to support custom ones? Custom ones are cool but there is not an obvious way to represent them in the UI, we might want to focus on key fields that we think are high value like Spencer is suggesting.