Add support for a footer row

[jcognard-actinvision]

Why

For one of my projects I need to add a footer row to my table and make it configurable by users to display sums, averages and more.
There’s already someone asking for a footer row support in #1208

What

This PR aims to enable support for a footer row by accepting a showFooter boolean attribute on DataTable component.
The content of the footer cells are defined with the footerContent function on the columns properties, which receives all visible rows as an argument, making possible to calculate sums, for example.

Example

import tableDataItems from '../constants/sampleDesserts';
import DataTable from '../../src/index';

const columns = [
	{
		name: 'Name',
		selector: row => row.name,
		sortable: true,
		fixed: true,
	},
	{
		name: 'Type',
		selector: row => row.type,
		sortable: true,
		fixed: true,
	},
	{
		name: 'Calories (g)',
		selector: row => row.calories,
		sortable: true,
		right: true,
		footerContent: rows => {
			const total = rows.reduce((acc, row) => acc + row.calories, 0);
			return Math.round(total);
		},
	},
];

export const Footer = () => {
	return <DataTable title="Movie List" columns={columns} data={tableDataItems} showFooter />;
};

I hope the quality of code matches your standards as it’s my first contribution.

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[jbetancur]

@jcognard-actinvision I've been a "little slow" to respond but I will take a look at this for consideration and review