multiple -i and -x fields should accumulate

[trentm]

Currently if -i FIELDS is used multiple times, then the FIELDS from the last one win. Ditto for -x FIELDS. I think they should accumulate (as I do in node.js with https://github.com/trentm/node-dashdash/blob/master/examples/custom-option-arrayOfCommaSepString.js). I.e. ecslog -i foo -i bar should include both "foo" and "bar" fields, not just "bar".