multiple `-i` and `-x` fields should accumulate

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".