enable multiple workers

[DaemonDude23]

Addresses #9 .

This is my first ever PR! Never written Ruby, but I pieced it together from other plugins who have enabled multiple workers.

References for this configuration property:

• https://docs.fluentd.org/v1.0/articles/performance-tuning-single-process#multi-workers
• https://www.fluentd.org/blog/fluentd-v0.14.12-has-been-released
• https://docs.fluentd.org/v1.0/articles/system-config#workers

[repeatedly]

Thx!