Added createBatchStream

[tinchoz49]

This PR:

• Adds a new method createBatchStream that works as createReadStream but the chunk returned on('data') is an array with the batch of messages. This feature reduce the time to get data and it can be compare to kappa.

$ browser-runner benchmark.js --timeout 0
# getBatch
ok ~33 ms (0 s + 33280000 ns)

# createReadStream [batch=1]
ok ~4.98 s (4 s + 983534999 ns)

# createReadStream [batch=100]
ok ~91 ms (0 s + 91230001 ns)

# createBatchStream [batch=100]
ok ~48 ms (0 s + 47880001 ns)

wins: getBatch
ok ~40 s (40 s + 81409999 ns)