Add nrows parameter to pandas.read_excel()
- [x] closes #16645 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I was working on this a few months back, but got busy. Apologies. I'm free until January so thought I'd pick this back up.

### Changes
* Added code for `nrows` parameter (with tests)
* Changed order of parameters to match `pandas.read_csv()` signature (across entire `pandas/io/excel.py` file)
* Increased readability by having one parameter per line
* Updated docstring to match new order (+ new entry for `nrows` param)

I think that covered all the outstanding issues from the [previous (closed) PR](https://github.com/pandas-dev/pandas/pull/16672). Please let me know if you need anything changed or modified.