[new] Nickel configuration language

Hi ports,

Here's a port of Nickel (https://nickel-lang.org/), a programmable
configuration language that allows both strong-typing and more dynamic
contract-driven specification of configurations.

There doesn't seem to be a uniform approach to building ports of
applications that happen to be distributed as Rust crates; I'm just
using the cargo index as SITES here.

Not hopping on MAINTAINER at this time, but maybe later.

Evan