Add `zgen completions` command

* Switch to using  `${ZGEN_COMPLETIONS}` array. This is more consistent with the way plugin files are handled. It will also made it easier to add the `zgen completions command`. `${ZGEN_INIT}` is also a lot tidier since we can just add `${ZGEN_COMPLETIONS}` to `fpath` in one line.
* Add `zgen completions` command. Some repositories contain completion functions, but aren't formatted as
standard zsh plugins, like `zsh-users/zsh-completions`, which makes `zgen load` error out when it can't find a plugin file. `zgen completions <optional subdirectory>` will clone a repo and add it (or the subdirectory) to $fpath.
* Document `zgen selfupdate` command.