Custom type with combined bigger/smaller is better

In addition to `customBiggerIsBetter` and `customSmallerIsBetter` it would be nice to just have a simple `custom` type where the JSON output would include `"better": "bigger"` or `"better": "smaller"` so that a single benchmark could make use of both. For example, I might want to measure both memory use (smaller is better) and throughput (bigger is better) for a single benchmark. While I could potentially save the results to a file and split them apart into two separate benchmark outputs, it would be simpler if this was not necessary.

EDIT: Having a quick look at the current code structure, I can see this probably isn't easy to do but it still would be nice :)