ENH: Add support for calculating EWMA with a time component
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


This enhancement allows EWMA to be calculated relative to the timestamp in which an observation occurs instead of assuming each observation is equally spaced.
Hello @mroeschke! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:











There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-07-06 21:33:14 UTC cc @pandas-dev/pandas-core if anyone has comments. thanks @mroeschke 