Support csv format for ArrayField

[roll]

Overview

Currently, it's only possible to express an array within e.g. a csv this way:

type: array
format: default

["v1", "v2", "v3"]  # note that it should be additionally quoted within csv

We'd like to support csv format to allow this:

type: array
format: csv

v1, v2, v3  # note that it should be additionally quoted within csv

Implementation

It's relatively simple to implement by updating fields.ArrayField and using builtin csv module of Python.

• write csv_cell_reader function in arrayField.create_cell_reader
• based on field.format: default | csv return cell_reader or csv_cell_reader
• add tests
• add docs

References

• Future possibility for delimiter-separated list for arrays (instead of JSON array)? specs#736.