Support for collections of values in individual cells

[arjun]

For follow up from connectathon discussion: Support for collections of values in individual output cells.

Questions to discuss:

• Are multi-value cells allowed? (Current thinking: yes, some implementations support this and may be useful for some use cases)
• Where should a parameter to indicate the desired behavior be set? In the ViewDef? In the Runner? (Suggestion: a command line flag to the Runner e.g. --csv-mode)
• What should be the default behavior?

TODO: document current behavior in the spec.