Add support of string split behavior compatible with python's `str.split`

This fix tries to address the issue raised in 18271 where
the existing `tf.string_split` does not match python's `str.split`.
Specifically, the `tf.string_split` does not handle the case where
separator might be multi-char.

This fix adds the implementation of string split compatible with `str.split`.
In order to maintain backward-compatible, this fix exposes the new
implementation of `array_ops.string_split_v2` into `tf.strings.split`
namespace.

This fix fixes 18271.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>