Add precision option to to_datetime_string()

[leafonsword]

to_datetime_string()'s result just to second precision:

>>> dt = pendulum.now()
>>> dt.to_datetime_string()
'2018-04-11 15:44:44'

Could to_datetime_string() add a option letting user choosing other precison such as millisecond、micosecond? While default precisipn still to be second ~

[sdispater]

The to_xxx_string() methods are supposed to be simple and with a predictable behavior and thus do not accept options. So unfortunately, I won't be adding a precision option for it.

If you want a more customizable behavior you can use format().