Table cells should support recursively add_table with style

[qzhua]

In the member function of Table._Cell

    def add_table(self, rows, cols):
        """Return a table newly added to this cell after any existing cell content,
        having `rows` rows and `cols` columns.

        An empty paragraph is added after the table because Word requires a paragraph
        element as the last element in every cell.
        """
        width = self.width if self.width is not None else Inches(1)
        table = super(_Cell, self).add_table(rows, cols, width)
        self.add_paragraph()

It would be nice if this function can support a style parameter.
Now we have to write code like this:

        word_table = cell.add_table(rows=0, cols=num_cols)
        word_table.autofit = True
        word_table.style = 'Table Grid'

What we want is :

        word_table = cell.add_table(rows=0, cols=num_cols, style = 'Table Grid')
        word_table.autofit = True

which would be consistent with the Document class