Support comma format

[nhairs]

The "comma" format (as mentioned in the cookbook), is probably worth adding as a directly supported format.

Pros:

• very simple to create from a list (or any iterable) using ",".join(x)
• simpler format to write so less prone to errors in the format
  • Other styles may have missing brackets, tokens, etc.
• very readable

Cons:

• This format is incompatible with other formatters.
  • i.e. once you move to this config it is harder to move to other formatters