croc CLI framework

This package is derived from github.com/schollz/cli/v2 v2.2.1, which in turn is based on urfave/cli and is distributed under the MIT license included in this directory.

It keeps the parsing behavior and the subset of the API used by croc. Help and fish-completion output are rendered directly instead of through text/template. Dynamic template method calls disable method dead-code elimination across a Go binary, so avoiding them materially reduces the size of both croc and croc-web without removing CLI features.