Warn on unused config options (#1402)
Warn on unused config options (#1402)

This will make it clear if a user has misspelled a config option, or if
an option has been changed/removed.
graydon/rust,Emilgardis/rustfmt,rust-lang/rustfmt,SingingTree/rustfmt,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,Emilgardis/rustfmt,SingingTree/rustfmt,graydon/rust,aidancully/rust,aidancully/rust,rust-lang/rustfmt,aidancully/rust,graydon/rust,graydon/rust,graydon/rust