unstable_features without CFG_RELEASE_CHANNEL
unstable_features without CFG_RELEASE_CHANNEL

Per discussion in #2228

https://github.com/rust-lang-nursery/rustfmt/pull/2228#issuecomment-348893294
https://github.com/rust-lang-nursery/rustfmt/pull/2228#issuecomment-349799548

Inline comment should explain the reasoning.

graydon/rust,graydon/rust,SingingTree/rustfmt,rust-lang/rustfmt,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,SingingTree/rustfmt,aidancully/rust,graydon/rust,aidancully/rust,rust-lang/rustfmt,aidancully/rust