Validate on all pod commands that the user is using a terminal in utf-8

Support a less vague pod pushing policy

Squash of https://github.com/CocoaPods/CocoaPods/issues/1510

Fix linter not throwing errors when a vendored library's path is invalid

Use `cocoapods` as CLAide#Command.plugin_prefix

To create a CocoaPods subcommand plugin gem, add the file
`lib/cocoapods_plugin.rb` and load your subcommand implementation from there.

When the `pod` command runs, it will look for that file in your gem and load
it before parsing the command line.

Corrected wrong output path passed to actool for Apps.

[CHANGELOG] CLAide plugins enhancement