Add apt_list_upgradable rule (#732)

This helps you run `apt list --upgradable` after `apt update`,
as it suggests.

README.md

@@ -281,6 +281,7 @@ Enabled by default only on specific platforms:
 * `apt_get` – installs app from apt if it not installed (requires `python-commandnotfound` / `python3-commandnotfound`);
 * `apt_get_search` – changes trying to search using `apt-get` with searching using `apt-cache`;
 * `apt_invalid_operation` – fixes invalid `apt` and `apt-get` calls, like `apt-get isntall vim`;
+* `apt_list_upgradable` – helps you run `apt list --upgradable` after `apt update`;
 * `brew_cask_dependency` – installs cask dependencies;
 * `brew_install` – fixes formula name for `brew install`;
 * `brew_link` – adds `--overwrite --dry-run` if linking fails;