Writing multiple data points per request

Instead of creating a separate request for each data point it would be nice if the library would allow sending multiple data points.

Somethink like
```
  influx.prepare(INFLUX_MEASUREMENT, tags, fields);
  influx.prepare(INFLUX_MEASUREMENT, tag, fields);
  influx.write();
```

See https://docs.influxdata.com/influxdb/v1.5/guides/writing_data/#writing-multiple-points