unstable features are usable on stable with the '--config' option

[bmwill]

If you place an unstable config in a rustfmt.toml config file and run
using a stable version of rustfmt the config is ignored and you get the
following:

$ cargo fmt
Warning: can't set `merge_imports = true`, unstable features are only available in nightly channel.

But if you instead pass the config using the '--config' option rustfmt
happily applies and uses the unstable config:

$ cargo fmt -- --config merge_imports=true

I don't know if this is intentional or not but it gave me a workaround for using the 'merge_imports' feature while using a stable toolchain: diem/diem#2926

[calebcartwright]

Yeah I noticed this the other day too (#4064 (comment)). I don't think it's intentional currently, and it's possible that cli config override support for unstable options on stable will be dropped as part of the rustfmt 2.x release

[calebcartwright]

@topecongiro are we planning to remove this (unstable opts on stable via --config) for the 2.0 release?