Improve handling of list option types

[imbriaco]

In order to set the value for options of type string, we have to pass the value as a comma separated string. For example, a command named foo with a list option named things would be called like this:

foo --things="one,two,three"

The things option that is passed to the command would then be set to [one two three].

This is not ideal either from the command user or developer point of view.

User

From the point of view of the user, it would be convenient if options of type list allowed the option to be specified more than once in the command invocation, and accumulated all the values. e.g.

foo --things one --things two --things three

The current comma separated list is also acceptable since that form remains useful in some cases, particularly when building a pipeline.

Developer

From the developer point of view, the value of an option of type list is marshalled into an unfamiliar format. I can see a couple of ways we could handle this:

1. Marshall options with type list as a JSON array, or
2. Use individual environment variables for the number of values for the option and the values, e.g.: COG_OPT_THING_COUNT and COG_OPT_THING_0, COG_OPT_THINGS_1, and so on.

It's not clear to me which of these is the "better" choice right now.

[imbriaco]

After thinking about it more, I lean toward the COG_OPT_THING_# style for passing lists of options for consistency with how we handle lists of args as well as simplicity of use.