Introduce self-update command like Composer does

[ruudk]

See #537, why do we have to copy installer scripts and run it. Feels unsafe and annoying on day to day.

Can we introduce a self-update command so that we can use castor self-update like Composer does?

[lyrixx]

I'm generally against it, but I'm changing my mind.

I was against it because it's difficult (many projects have abandoned this feature).

But Composer is capable of doing it, so I'm all for giving it a try.