Facilitate autocomplete for add, substrac methods

[hongquan]

Currently (v0.6.9), OffsetDateTime's add() and subtract methods require explicitly passed ignore_dst=True parameter. I think if it is required and accepts only one value (True), then it is better to meld it to method name. By doing so, we don't have to trigger editor autocomplete 2 - 3 times. The method name could be add_ignoring_dst, subtract_ignoring_dst.