Allow column_names to be a method in Table.__init__

[jpmckinney]

If a method is passed as column_names, this method should take the number of columns as an argument, and return column names. For example, if I wanted Excel columns like 'AA,AB,AC' instead of agate columns like 'aa,bb,cc', I would be able to pass it this method:

def excel_column_names(n):
    column_names = []
    for col_idx in range(1, n + 1):
        # From openpyxl's cell.py
        letters = []
        while col_idx > 0:
            col_idx, remainder = divmod(col_idx, 26)
            # check for exact division and borrow if needed
            if remainder == 0:
                remainder = 26
                col_idx -= 1
            letters.append(chr(remainder + 64))
        column_names.append(''.join(reversed(letters)))
    return column_names

[jpmckinney]

I'm abandoning the feature request.