Add factorial.

src/algorithms/math/factorial/README.md

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+# Factorial
+
+In mathematics, the factorial of a non-negative integer `n`, 
+denoted by `n!`, is the product of all positive integers less 
+than or equal to `n`. For example:
+
+```
+5! = 5 * 4 * 3 * 2 * 1 = 120
+```
+
+| n     | n!                          | 
+| ----- | :-------------------------: |
+| 0     | 1                           |
+| 1     | 1                           |
+| 2     | 2                           |
+| 3     | 6                           |
+| 4     | 24                          |
+| 5     | 120                         |
+| 6     | 720                         |
+| 7     | 5 040                       |
+| 8     | 40 320                      |
+| 9     | 362 880                     |
+| 10    | 3 628 800                   |
+| 11    | 39 916 800                  |
+| 12    | 479 001 600                 |
+| 13    | 6 227 020 800               |
+| 14    | 87 178 291 200              |
+| 15    | 1 307 674 368 000           |
+
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Factorial)

src/algorithms/math/factorial/__test__/factorial.test.js

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+import factorial from '../factorial';
+
+describe('factorial', () => {
+  it('should calculate factorial', () => {
+    expect(factorial(0)).toBe(1);
+    expect(factorial(1)).toBe(1);
+    expect(factorial(5)).toBe(120);
+    expect(factorial(8)).toBe(40320);
+    expect(factorial(10)).toBe(3628800);
+  });
+});

src/algorithms/math/factorial/factorial.js

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+/**
+ * @param {number} number
+ * @return {number}
+ */
+export default function factorial(number) {
+  let result = 1;
+
+  for (let i = 1; i <= number; i += 1) {
+    result *= i;
+  }
+
+  return result;
+}