Add support for interquartile range

[boristane]

Add support for interquartile range as defined here.

One should be able to provide the quartiles if already computed and avoid re-calculating them.

iqr([7, 31, 7, 31, 47, 75, 87, 115, 116, 119, 119, 155, 177]); // -> 88
iqr([7, 31, 7, 31, 47, 75, 87, 115, 116, 119, 119, 155, 177], [31, 87, 119]); // -> 88

Update README.md under Averages and measures of central location and the table of contents.