Introduce proc macros for scraper

[LoZack19]

As of now, the scraper proc macros are an independent crate, and they don't constitute an optional feature, which is what we want to achieve. Moreover the selector macro replicates parsing both at runtime and at complie time, because the objects from selectors crate don't implement quote's ToToken trait, which could allow us to adopt a more efficient approach. For all this reasons, I'll mark this as a draft for now.

[cfvescovo]

because the objects from selectors crate don't implement quote's ToToken trait, which could allow us to adopt a more efficient approach

I think we should wait for this to be implemented before providing an "official" proc macro