ENH: Added ability to freeze panes from DataFrame.to_excel() (#15160)
 - [X] closes #15160
 - [x] tests passed- Existing tests passed. New test added and passed.
 - [X] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry- Added to 0.20.

The ability to freeze panes has been added to the to_excel() method on data frames. The parameter is a tuple of 2 integers, representing the bottom-most row and right-most column to freeze, respectively. (This is the same format xlsxwriter uses). The default value is None, resulting in no freezing.

Example usage: 
``df.to_excel(writer, sheet_name='Sheet1', freeze_panes=(1,1))``

The following engines are supported:
xlsxwriter
xlwt
openpyxl22

The parameter exists for openpyxl20 and openpyxl1 but it has no effect, in anticipation of support for these versions being dropped (#15184).

Can one of the maintainers confirm this approach for engine support is adequate, or suggest an alternative? Once that is settled I can write a test to create a workbook using each engine and then try reading it back in using openpyxl.
