Split Spress into Spress.Core and its ecosystem

[yosymfony]

In short, split Spress into:

• Spress Core.
• Ecosystem: CLI command, built-in server, scaffolding and others.

Namespaces:

• Spress Core: Yosymfoy\Spress\Core
• Rest: Yosymfony\Spress

Backward compatibility:

For backward compatibility purposes events of plugins and plugins class base are located in namespace Yosymfony\Spress\Plugin. This situation end with Spress 2.0.0.