Update default edition to 2018

[dtolnay]

The current behavior of rustfmt as of 8ec0750 is:

$ echo 'use ::serde::Serialize; mod serde {}' | cargo run --bin rustfmt

use serde::Serialize;
mod serde {}

That is, by default the rustfmt cli command is assuming edition 2015. This assumption actually breaks the meaning of some 2018 style imports as seen in #3501.

Quoting from @topecongiro in #3501 (comment):

As the default edition of rustc and cargo are both 2018, I think it's reasonable to change the default edition of rustfmt to 2018 in 2.0.