add a trigger to puma config

- Remain the same config output for minor changes
- `set :puma_active_record_establish_connection, true` to enable this
  feature
prepare for v0.2.0

- rename :puma_active_record_establish_connection to
  :puma_init_active_record
Merge pull request #26 from crhan/AREC

add a trigger to puma config, for support Issue #25