DOC: Exposed arguments in plot.kde
The documentation for `plot.kde` did not show the `bw_method` and `ind` arguments, which are specific to `plot.kde` (and `plot.density`, which refers to the same method).

There is also a change in the actual code, and a corresponding test added. I added an option for `ind` to be an integer number of sample points, instead of necessarily a `np.array`. I image that a user typically just wants a number of equidistant sample points, and does not want to construct an array.

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

Comments very welcome.
