General refactor

- removes command completion in favor of snap versioning.
- simplifies installation by defaulting to Homebrew install directories and ~/.phps if they exist.
- re-organize readme deferring some of the optional configuration and help info to the wiki and to `php-version --help`.
- better variable names.
- better language around features, who this is for and who it is not for.
- favor `homebrew-php` installations over manual install or `php-build`.
- bumps version to 0.9.9.