Add plugin autoregistration support

[ereOn]

With the upcoming support for plugins (#3), the resulting .plix.yml configuration file might end-up too crowded buy the long plugin names.

We need to allow plugins to automatically register to plix with an alias so that the user can use just an alias in the .plix.yml file.

Example:

plugins: [python, ruby, c++]

These aliases can never contain a . (otherwise a legacy Python import-style lookup is done instead).

To implement this mecanism, we need plugins to be registered somehow when plix starts.

Here is a related interesting read.