Add a QUIET feature

[asensio-project]

Hello,

I think that would be good idea adding a cargo feature for avoiding the bootloader to print messages in the screen until loading the kernel.
This way we don't need to clear the screen before printing messages.

Thanks.

[Freax13]

Note that you might have to clear the screen anyways because some BIOS' will show the manufacturers logo on startup.

[asensio-project]

I opened a PR for adding the feature.
#292

[asensio-project]

I published a new PR #303.
It's ready for review.