function linearRegressionLine(mb /*: { b: number, m: number }*/) {
    // Return a function that computes a `y` value for each
    // x value it is given, based on the values of `b` and `a`
    // that we just computed.
    return function (x) {
        return mb.b + mb.m * x;
    };
}
Given the output of `linearRegression`: an object
with `m` and `b` values indicating slope and intercept,
respectively, generate a line function that translates
x values into y values.

@param {Object} mb object with `m` and `b` members, representing
slope and intersect of desired line
@returns {Function} method that computes y-value at any given
x-value on the line.
@example
var l = linearRegressionLine(linearRegression([[0, 0], [1, 1]]));
l(0) // = 0
l(2) // = 2
linearRegressionLine({ b: 0, m: 1 })(1); // => 1
linearRegressionLine({ b: 1, m: 1 })(1); // => 2