Allow 3rd party plugins

Merging this pull request will allow 3rd party plugins to be loaded in a project's phpci.yml file using the following syntax:

```yml
test:
    \Vendor\NameSpace\3rdPartyPlugin:
        argument: "Yes"
    php_mess_detector:
    php_code_sniffer:
        standard: "PSR2"
    php_loc: ~
````

Any plugins loaded in this way will need to be loadable by PHPCI's autoloader. The easier way to do this would be to register them using composer.