Compatible with pongo2/v4

[kataras]

Hello @flosch, three days ago you published the pongo2 v4 however, this pongo2-addons are incompatible with that version. I've made the necessary changes in my pongo2-addons fork's branch v4.0.0. But, I cant publish to master because it will break the compatibility for the old users, that's why I am recommending you to create a new branch: v4 or v4.0.0, so I can push my PR here, and then you can just push a release of v4.0.0, the new import path will be:

github.com/flosch/pongo2-addons/v4 

will work for pongo2/v4

And the existing github.com/flosch/pongo2-addons will still working with pre-v4 users of pongo2.