Add option to choose fields to be displayed

[mlimaloureiro]

When doing an ec2 ls for instance it would be nice to have an option to choose the fields we want to see, instead of using a grep

[donnemartin]

@mlimaloureiro interesting idea. Curious how the user would select which fields to display...perhaps an autocomplete or to show help in the completer or in-line?

In addition to grep, I think jq is also handy for filtering the output, if it's set to json.

The docs for awscli suggest something like this to show just the instance ids, which SAWS might make more convenient:

aws ec2 describe-instances --query "Reservations[].Instances[].[InstanceId]