Mark certain nodes to be kept: `crux-keep`

[chimbori]

See the excellent commentary by @sigpwned on #14

[guareber]

Would it make sense to extend the feature so that crux-keep=false would override in the opposite direction? I know I'd certainly find that useful, I'm just not sure if you'd agree it falls within the scope of the project and that crux-keep would be the mechanism to implement it.