ENH: Allow referencing of Excel columns by their Excel column names.
I find it more convenient to refer to the columns as they are in Excel by their string representations.

The excel2num() function I found on StackOverflow. Does that need to be referenced/acknowledged?

I'm happy to put in some unit tests but couldn't find a natural home for it. Most of the ExcelFile tests sit in test_frame.py but I think something like test_io.py or test_parsers.py would be a better fit.
