Shell completion

CakePHP offers a great console granting us the tools to build all kinds tools. While there is a lot of meta data available we can't use it for bash completion without a plugin, [@AD7six/cakephp-completion](https://github.com/AD7six/cakephp-completion). This PR is based on that plugin, with the authors consent.

The reason I'm submitting this PR is because using that plugin in my projects has made life a lot easier when dealing with shell commands. Due to it simple nature and easy implementation on the users end (just copy cake.bash to the correct directory and reload your shells profile to get it work work). It is in my opinion a great addition to the framework.

If this PR is merged I'll also make sure documentation is added to the cookbook and the functionality is ported to 3.0.