Update message to more closely align with stock

The regular failure message is

nodenv: version `12' not installed

We can match that a bit better with:

package-json-engine: version satisfying `12' not installed

Don't emit warnings for unreadable or non-file package.json

Quiet errors from JSON.sh

Credit JSON.sh and link to docs for options

Skip aliases

No need to select aliases since the aliased node would already be included itself.

Handle non-semver node versions

sh-semver emits warnings when non-semver strings are passed to it.
So we should filter out and ignore any node versions that don't at least
start with a number.

This could be a lot smarter, and currently only works because node
versions omit the implicit 'node-' prefix.

Minor spelling fix

Use JSON.sh to parse package.json

This replaces a regex that could match babel-preset-env settings.

Fixes #31

Switch to version hooks. Fixes #25.

Using version-name and version-origin instead of the which
supports more nodenv commands, includeing nodenv version.

Full explanations of hooks here:
https://github.com/rbenv/rbenv/wiki/Authoring-plugins#rbenv-hooks

Extract plugin_root script

To enable moving shared functions into libexec.

Reorganize functions in libexec

Prompted by the need to share precedence checks betweeen version-name
and version-origin, I've extracted mosted of the package-json-engine
logic into functions in libexec.

Also fixes #10.

Rebase nodenv-shell precedence onto version support

With a slightly different check for nodenv shell that lets us still
respect shell version even when it matches the global version.

Lint with Shellcheck

And run automatically on travis

Switch to depending on semver from npm

Fix script dir lookup to handle symlinks

Refactors script dir lookup into a function

Functions then 'main'

Search for installed versions matching semver range

shellcheck fixes

safe array handling of nodenv versions