Add WRITE_TRUNCATE_DATA write disposition

Hello,

BigQuery Job has now 4 write dispositions : 
WRITE_EMPTY, WRITE_TRUNCATE, WRITE_APPEND and WRITE_TRUNCATE_DATA

Currently in pandas-gbq WRITE_EMPTY, WRITE_TRUNCATE, and WRITE_APPEND are implemented in pandas_gbq.to_gbq(). 
==> Is it possible to add WRITE_TRUNCATE_DATA to pandas_gbq.to_gbq() ?

Best regards,

JMCarthy