Add batch-put benchmark

[vweevers]

npm i leveldown
level-bench run batch-put leveldown
level-bench run batch-put leveldown -b [--chained]
level-bench run batch-put leveldown -b [--batchSize 500]
level-bench run batch-put leveldown -b [--batchSize 500 --chained]
level-bench run batch-put leveldown -b [--batchSize 10]
level-bench run batch-put leveldown -b [--batchSize 10 --chained]
level-bench plot batch-put