ssh-key: add a crate feature to allow insecure RSA keys - fixes #336#337

Eugeny wants to merge 2 commits intoRustCrypto:masterRustCrypto/SSH:masterfrom

Eugeny:ssh-key-hazmat-allow-insecure-rsa-keysEugeny/RustCrypto-SSH:ssh-key-hazmat-allow-insecure-rsa-keysCopy head branch name to clipboard