Warn on space in modifiers instead of failing?

[taion]

Not an actual problem right now given that #19 has been reverted, but it turns out that Prettier currently always strips out these spaces: https://github.com/prettier/prettier/issues/4492#issuecomment-391342006

I almost missed this... given this, maybe for v2, as for using color instead of e.g. color-mod, perhaps it would be better to warn on the spaces?