wip: Initial import of Elvish

[Michael-John Turner <mj%mjturner.net@localhost>]

Hi all,

I've just imported v0.15 of Elvish into wip.

"Elvish is an expressive programming language and a versatile interactive
shell, combined into one seamless package. It runs on Linux, BSDs, macOS
and Windows."

As this is both my first Go package and first shell, feedback is welcome :)
I've tested on NetBSD and macOS.

[1] https://elv.sh/

Cheers, MJ
--
Michael-John Turner * mj%mjturner.net@localhost * http://mjturner.net/