ENH: fill_value argument for shift #15486
This is a work in progress. 

Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):

- [X] closes #15486 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @samuelsinayoko! Thanks for updating the PR.













Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on April 29, 2018 at 10:46 Hours UTC what issue is this for? can you fix the title This is work in progress. Current implementation doesn't work for categorical data, hoping to fix that soon.  Hi Samuel Sinayoko (@samuelsinayoko),
We are researchers working on identifying redundant development and duplicated pull requests. We have found there is a pull request: https://github.com/pandas-dev/pandas/pull/15527 which might be a potentially duplicate to this one. We would like to build the link between developers to reduce redundant development. We would really appreciate if you could help us to validate and give us feedback.
Thank you very much for your time!
 closing as stale, if you want to continue working, pls ping and we can re-open. you will need to merge master.