Make nox hackable/extendable

[Nostradamos]

Currently it's not possible to extend nox once it's imported. Would be good to change that. Ideas:

• hooks to hook into gulp-data and stuff
• _config.yml which allows to define hooks, new filter methods and stuff like that
• a way to modify gulp tasks? Maybe write a overlay over the gulp.task gulp.series gulp.src stuff which represents the hierachy as a object/tree similiar to gulp.tree.

[Jikstra]

• a way to modify gulp tasks? Maybe write a overlay over the gulp.task gulp.series gulp.src stuff which represents the hierachy as a object/tree similiar to gulp.tree.

Thought a bit about this, i think the best way would be to have a seperate npm module which allows us to define all gulp tasks as a modifyable tree (some kind of object). On a specific hook we could modify this tree and after this hook is finished, the tree is getting compiled down to the normal gulp stuff. Basically it's another layer on top of gulp, which allows heavy modification (adding in new methods/tasks, swapping out tasks..., modifying tasks, even removing tasks) to do all the crazy shit people maybe want to do. But as this still gets compiled to normal gulp task stuff, it will still allow us to just use the gulp-cli.