ENH: add option to tz_localize to return NaT instead of raising a NonExistentTimeError
It would be nice if the `tz_localize` function of a `DatetimeIndex` had an optional flag for silently returning `NaT` instead of throwing a `NonExistentTimeError`, if the timestamp is not valid in the given timezone (for example due to DST changes).
I ran into this problem while trying to `tz_localize` a large index, and it seems to me that this would be a much better solution than manually handling the exception with a lambda expression in a (slow) python loop.
