Adds an option to set a custom directory for the fonts folder.

[EngKhaledB]

The issue:

For read-only hosting like Pantheon for example, they prevent php from writing any file outside uploads directory.
So that, a lot of php errors happens because of no write permissions.

The Fix:

I added an option to set a custom fonts directory via constant variable named KIRKI_FONTS_DIR.

[contactjavas]

Hi @EngKhaledB , good suggestion!
I'm not sure if we should use constant or filter for that. What do you think @mapsteps ?

Oh, maybe we forgot to mention that this repo contains individual controls.
The real developments are in the individual repositories.
So this PR should be in kirki-framework/module-webfonts repository.

Thanks :)