NEW: shells/elvish

Hi ports --

Attached is a new port, shells/elvish. Elvish is a cross-platform,
friendly, and expressive shell. It's written in Go, and so I used this
to get familiar with porting Go applications. But it seems fairly
popular on GitHub and works so I'm sending in the port.

It feels somewhat similar to fish, which makes sense as the Elvish
people say that fish partly inspired their design philosophy.

---
pkg/DESCR:
Elvish is a cross-platform shell. It features an expressive programming
language, with features like namespacing and anonymous functions, and a
fully programmable user interface with friendly defaults. It is suitable
for both interactive use and scripting.
---

OK?

~Brian