added utilities and flags --fail-message and --fail-with

non-GNU `paste` support in `defn_terse`

non-GNU `head` support in `defn_terse`

made success messaging dependent on installation exit code when using…

… `icli::always_install`

enabled use of `require()` for no-install (check-only) checks, adding…

… `--long-option=...` syntax to its signature, prefixed all library entities to avoid polluting the shared global namespace, added post-install check to confirm success, and added keyboard-interrupt trap clean-up

added example implementation script and documentation

factored out more boilerplate

initial release