ENH: gb.is_monotonic_increasing #17015
- [x] closes [#17015](https://github.com/pandas-dev/pandas/issues/17015#issuecomment-316238679)
- [x] tests added / passed: 
updated test_tab_completion to expect additional functions `is_monotonic_increasing` and `is_monotonic_decreasing` 
added (semi-redundant) tests of gb.is_monotonically_increasing() - strictly increasing/decreasing, non-strictly, w/ inf vals, w/ nan vals
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
added at the end of the `Other Enhancements` section

This is my first PR to Pandas, so I don't doubt that I'll need to edit things.