Overhaul how Config is passed around

Overhaul how Config is passed around

This commit started out by moving the hash map of configuration values into
`Config` and then removing all calls to config::all_configs in favor of taking a
`&Config` argument. This initial step was taken to prevent reloading
configuration many times from disk, but it also provides a nice house for
configuration to be loaded from.

The commit snowballed into purging `MultiShell` as an argument all over the
place and passing around `Config` instead. Along the way a few other breaking
changes were made:

* The `config_for_key` and `config_list` subcommands have been removed. These
  commands are not ready to be stabilized and need some more work. In general
  the `cargo config` system needs to be cleaned out and built back up with a new
  interface, at which point these subcommands can return.
* The old and deprecated behavior of `cargo update foo` has been removed.