Extracted blog from antimatter and rearrange it as a plugin

Hello,
I've just commited a new plugin which simply contains the Antimatter blog features, to easily add blog features to all themes.

This is just a proposal because it involves something that already works very well in Grav. For these reasons, I'd like to ask you to partecipate to the discussion, if you are interested.

Here's the issue link to request adding this plugin: https://github.com/getgrav/grav/issues/256.