Add urls to every diagnostic

[Jake Shadle]

Similarly to clippy eg.

error: `assert!(true)` will be optimized out by the compiler
  --> src/lib.rs:13:5
   |
13 |     assert!(true);
   |     ^^^^^^^^^^^^^
   |
   = help: remove it
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assertions_on_constants

every diagnostic emitted by cargo-deny should have a url that points to helpful documentation.