stats: add new stats to stats line (#166)
This adds "Stats.ShardsScanned" and "Stats.ShardsSkippedFilter" to the stats line.
```
Took 2.209097ms for 5208B index data, 3153 ngram matches, 138 docs considered, 138 docs (4027KB) loaded, 39 shards scanned, 4 shards filtered, 27368 docs skipped, 0 shards skipped.
```