Add Polka (Node.js)

[lukeed]

Adds a normal Polka app as well as Polka running within a cluster.

Also modified .gitignore to hide all node_modules by default~

FWIW, I didn't shut down everything for a clean environment, but my results nonetheless:

OS: Darwin (version: 17.4.0, arch: x86_64)
CPU Cores: 8
---

| Language (Runtime)        | Framework (Middleware)    |       Max [sec] |       Min [sec] |       Ave [sec] |
|---------------------------|---------------------------|-----------------|-----------------|-----------------|
| node                      | express                   |       14.045810 |       13.786849 |       13.860336 |
| node                      | clusterexpress            |        6.542760 |        6.045073 |        6.294131 |
| node                      | polka                     |        7.428920 |        7.351898 |        7.406287 |
| node                      | clusterpolka              |        3.814742 |        3.681836 |        3.740319 |

---

Closes #104

[tbrand]

Awesome!:tada:

[kalicki]

@tbrand update result in readme?!

[tbrand]

Sorry, see #102

[tbrand]

Updated 🎉
https://github.com/tbrand/which_is_the_fastest/releases/tag/v1.2.3