Proposal for changing --region/--ignore-checks/--append-rules argument format

This proposal is following discussions in #166 and elsewhere.

Current behaviour for -r/-i/-a args is essentially argparse nargs='*', which means you can specify zero or more values for them. This is a bug, and it should be nargs='+', so you can specify one or more values.

Example of currently allowed commandlines:

# Implicit separation between exhaustive args and positional arg
cfn-lint -r us-west-1 us-west-2 -i E101 E102 -a E201 E202 -d template.yaml
# Explicit separation
cfn-lint -r us-west-1 us-west-2 -i E101 E102 -a E201 E202 -- template.yaml
# No positional arg
cfn-lint -r us-west-1 us-west-2 -i E101 E102 -a E201 E202 -t template.yaml

Proposed behaviour is to change -r/-i/-a to support single value (of comma-separated items) -r us-west-1,us-west-2 but allow multiple specification -r us-west-1 -r us-west-2.

Example of proposed commandlines:

# Form 1
cfn-lint -r us-west-1,us-west-2
# Form 2
cfn-lint -r us-west-1 -r us-west-2
# Both together is valid
cfn-lint -r us-west-1,us-west-2 -r us-west-3

[cmmeyer]

I find use case #3 horrifying, but see no reason not to support it. I like the comma separated list better than spaces personally – it parses better to humans as well. 👍

[cmmeyer]

Hey, are we good for this one at this point? I think we’ve got command line args mostly sorted at this point. I’m going to go ahead and close this issue if no one has any objections…