Lint warnings should explain how to disable them

[brson]

The first time a warning trips it should emit a note explaining which lint setting caused it and possibly how to change it.

[sammykim]

I am working on this.

I will follow GCC for formatting. That is:

test.rs:2:5: 2:8 warning: unused variable: `var` [-Wunused-variable]
test.rs:2       let var = 1;
                    ^~~