[ARGV] Rename to coerce.
[Command] There is no need for separate option module.

A client needs to be able to easily subclass any base Command
code, pushing this out into different modules doesn't help.
[Command] Add paranoid guard for if an exception is raised before a command being parsed.
[PluginManager] Add support for loading multiple plugin prefixes.

Also rename from PluginsHelper.
[PluginManager] Doc fix.
[Command] By default add the `clad` plugin prefix.

This is where the completion script generator can move to.
E.g. `claide-zsh-completion`.
[ShellCompletionHelper] Remove.

Fixes #43.

Should be moved to a plugin instead.
[Command] Rename `handle_root_options` back.

To be inline with the constants.
Remove last mentions of completion script.