Rewrite npm's scripts in JavaScript

[aminya]

• This removes the dependency on sh for a Node env, which is not available on Windows.
• It also speeds up the startup time because there is no sh script anymore to slow things down. Nodejs itself spawns the executable directly.

Fixes #183
Related to #16

[aminya]

@DmitryTsepelev Could you take a look at this? It makes it possible to use lefthook on Windows.

[Envek]

@aminya hello and thanks for the pull request.

We're improving and maintaining Lefthook in the free time from commercial work, so we can't react to all contributions promptly.

We will get to your PR some day, but in the meantime please be patient and don't mention random people in the team. Thank you for understanding.

[aminya]

You're welcome. I am also contributing in my free time from my commercial work, and since the mentioned member approved two pull requests today, I added one kindly reminder. In the meantime, I am using my local fork, and I am not in rush at all.

[Envek]

Windows support is especially hard to do right as we don't have anyone who is on Windows. That's why your contribution is especially valuable. @aminya, thank you very much!

[Envek]

Released in 0.7.5. Thank you!

[aminya]

You're welcome!

[PikachuEXE]

This change (precisely the removal of .npm/bin/lefthook) causes #232.
Would be good to hear your opinion on how to fix #232. (Probably better to discuss there)