Provide mechanism for setting ignoreCompilationError

[marienz]

See #50: when using package! to add an additional package not in nixpkgs/emacs-overlay, our generated derivation treats byte-compilation failures as fatal. Mostly because Doom doesn't do that, it would be nice to provide an easy opt-out. Currently, I think you'd have to write a derivation by hand.

I don't really want to default this to off since it catches real bugs when applied to packages depended upon by Doom.

One possible approach is a setting to disable it for the "user" module (which should be doable, doom-intermediates/packages.json has a set of modules for each package).

We could also do something a bit more fine-grained: I've been meaning to add support for something overlay-ish layered on top of our generated derivations...