Improve bash completions

We already have tab completions for command and subcommand names. We don't have:

* completions for associative params for each command
* completions for some positional params, such as plugin slugs etc.

I'm thinking something like `WP_CLI::add_completion( 'plugin', 'Plugin_Command_Complections' );`

TODO:

* [ ] fourth level commands don't show up at all
* [ ] `wp bp core` completes with `wp core` subcommands
* [ ] complete with filenames for certain commands (fixes #994)