Do not require comma delimiters for exec form

[mthalman]

In order to better facilitate parsing of the output from the docker history command, it's necessary to loosen the requirements of parsing exec form instructions such as CMD.

As an example, a Dockerfile containing the instruction CMD ["echo", "Hello"] will be represented as CMD ["echo" "Hello"] in the output of the docker history command (notice the lack of a comma). So it's not truly a valid JSON array syntax.

This updates the parsing logic to allow the comma to be optional in this case.