Add benchmarking task

Add benchmarking task. (ref. #12)

`rake benchmark:vector` to benchmark `Yeah::Vector`.

```sh
$ rake --tasks
rake benchmark:vector  # Benchmark Yeah::Vector
rake web:test          # Run tests through opal-minitest
$ rake benchmark:vector
                 user     system      total        real
+            0.080000   0.000000   0.080000 (  0.077147)
-            0.080000   0.000000   0.080000 (  0.081059)
*            0.070000   0.000000   0.070000 (  0.071861)
/            0.080000   0.000000   0.080000 (  0.076522)
+@           0.060000   0.000000   0.060000 (  0.065507)
-@           0.100000   0.000000   0.100000 (  0.100169)
length       0.060000   0.000000   0.060000 (  0.051005)
distance_to  0.100000   0.000000   0.100000 (  0.108767)
angle_to     0.150000   0.000000   0.150000 (  0.147239)
along        0.110000   0.000000   0.110000 (  0.109092)
along!       0.070000   0.000000   0.070000 (  0.070629)
```

Iterations are able to set via environment variable (default: 100,000).
```sh
$ ITERATIONS=1_000_000 rake benchmark:vector
                 user     system      total        real
+            0.710000   0.000000   0.710000 (  0.714035)
-            0.730000   0.000000   0.730000 (  0.725722)
*            0.680000   0.000000   0.680000 (  0.682436)
/            0.680000   0.000000   0.680000 (  0.687387)
+@           0.600000   0.000000   0.600000 (  0.597083)
-@           1.000000   0.000000   1.000000 (  1.003476)
length       0.520000   0.000000   0.520000 (  0.517457)
distance_to  1.050000   0.000000   1.050000 (  1.051363)
angle_to     1.420000   0.010000   1.430000 (  1.424797)
along        1.040000   0.000000   1.040000 (  1.043859)
along!       0.650000   0.000000   0.650000 (  0.653981)
```

yeah?