Allow populating CliUnstable from a HashMap
Allow populating CliUnstable from a HashMap

This makes it easier to populate unstable
options from configuration files. Might also
help make unstable option tests easier to write.

rust-lang/cargo,tshepang/cargo,eminence/cargo,tshepang/cargo,rustyhorde/cargo,rust-lang/cargo,tsurai/cargo,tshepang/cargo,tshepang/cargo,tsurai/cargo,nipunn1313/cargo,rustyhorde/cargo,rust-lang/cargo,rustyhorde/cargo,tsurai/cargo,nipunn1313/cargo,eminence/cargo,tsurai/cargo,nipunn1313/cargo,eminence/cargo,rustyhorde/cargo,rust-lang/cargo,tsurai/cargo,nipunn1313/cargo,eminence/cargo