Would like to contribute parse time operator into tfa.text

[helinwang]

Hi SIG Addons community,

I would like to contribute parse time operator into tfa.text.

Here is the short description (detailed description in the attachment):

The parse time operator parses a timestamp string with the given format into the Unix time representation, the number of seconds / milliseconds / microseconds / nanoseconds elapsed since January 1, 1970 UTC.
For example, “2019-05-17T23:56:09.05Z” with the format “%Y-%m-%dT%H:%M:%E*S%Ez” gets parsed into 1558137369, when the output unit is set to SECOND.

For more details please see:
Parse Time Operator.pdf

I would be willing to maintain the Op going forward.

[seanpmorgan]

Hi @helinwang as mentioned on the mailing list -- I think this would be a nice addition. Could you go ahead with a PR and we can review there.

[helinwang]

Thanks, Sean!
I sent #530.
Btw, the docker based development / build environment that tensorflow/addons provides is pretty convenient to use :)