Use custom-head when Minima 3 is released

[FernandoBasso]

When Minima 3 is released, we could (should?) use custom-head.html to extend the default head.html (instead of completely overriding it).

References:

• https://github.com/jekyll/minima#extending-the-head-
• 2.5.1 can't include custom-head.html jekyll/minima#472

[mattrose]

@FernandoBasso I think this is released now if you want to fix it up.

[FernandoBasso]

@mattrose Cool. I'll check it out asap!