Added Extended Euclidean Algorithm

src/algorithms/math/extended-euclidean-algorithm/README.MD

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+# Extended Euclidean algorithm
+
+In arithmetic and computer programming, the extended Euclidean algorithm is an extension to the Euclidean algorithm,
+and computes, in addition to the greatest common divisor of integers a and b, also the coefficients of Bézout's 
+identity, which are integers x and y such that `ax + by = gcd(a,b)`.
+This is a certifying algorithm, because the gcd is the only number that can simultaneously satisfy this equation and 
+divide the inputs. It allows one to compute also, with almost no extra cost, the quotients of a and b by their 
+greatest common divisor.
+
+![Extended](https://i.stack.imgur.com/UiQ2m.png)
+
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm)

src/algorithms/math/extended-euclidean-algorithm/__test__/extendedEuclid.test.js

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+import extendedEuclid from '../extendedEuclid';
+
+describe('extendedEuclideanAlgorithm', () => {
+  it('should calculate coefficients', () => {
+    expect(extendedEuclid(30, 20)).toEqual({
+      gcd: 10,
+      x: 1,
+      y: -1,
+    });
+
+    expect(extendedEuclid(30, 50)).toEqual({
+      gcd: 10,
+      x: 2,
+      y: -1,
+    });
+
+    expect(extendedEuclid(65, 40)).toEqual({
+      gcd: 5,
+      x: -3,
+      y: 5,
+    });
+  });
+});

src/algorithms/math/extended-euclidean-algorithm/extendedEuclid.js

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+/**
+ * @param {number} Coefficient A
+ * @param {number} Coefficient B
+ * @return {Object} {gcd, x, y}
+ */
+
+export default function extendedEuclid(coeffA, coeffB) {
+  if (coeffA === 0) {
+    return {
+      gcd: coeffB,
+      x: 0,
+      y: 1,
+    };
+  }
+  const variables = extendedEuclid(coeffB % coeffA, coeffA);
+  return {
+    gcd: variables.gcd,
+    x: (variables.y - Math.floor(coeffB / coeffA) * variables.x),
+    y: variables.x,
+  };
+}