Introduce workspaces

[LoZack19]

This PR aims to introduce workspaces in scraper, so that it is possible to provide scraper_proc_macros in a separate crate as an optional feature, as suggested in issue #188. Before introducing this change you need to make sure that it does not create problems. It would probably be appropriate to symlink the license and readme within the scraper crate, as other crates do, but since I work on windows I wasn't sure weather it would be a great idea, since links work differently between windows and linux, and probably many people use the latter.

This changes the directory structure from:

.
├── Cargo.toml
├── Cargo.lock
├── src/
├── examples/
├── README.md
├── scraper.1
└── LICENSE

To:

.
├── Cargo.toml
├── Cargo.lock
├── scraper/
│   ├── src/
│   ├── examples/
│   ├── Cargo.toml
│   └── Cargo.lock
├── README.md
├── scraper.1
└── LICENSE

[LoZack19]

This breaks the crate at the moment. Running cargo publish --dry-run -p scraper I get the following error:

error: couldn't read `src\../../scraper.1`: Impossibile trovare il file specificato. (os error 2)
  --> src/main.rs:90:27
   |
90 |         fs::write(&path, &include_bytes!("../../scraper.1")[..]).unwrap();
   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this error originates in the macro `include_bytes` (in Nightly builds, run with -Z macro-backtrace for more info)
help: there is a file with the same name in a different directory
   |
90 |         fs::write(&path, &include_bytes!("../../../../scraper.1")[..]).unwrap();
   |                                          ~~~~~~~~~~~~~~~~~~~~~~~

error: could not compile `scraper` (bin "scraper") due to 1 previous error
error: failed to verify package tarball

[cfvescovo]

Fixed

[LoZack19]

Perfect, it works. I would merge this.