default.nix: Allow to customize build options
default.nix: Allow to customize build options

Sometimes I just want to see the generated JS code of a particular
application and don't want to compile the examples or even run/compile
the tests.

This makes it possible to pass arguments to the build function.

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@redmoonstudios.org>

aszlig/hase