Add hashes and nonces as friendly directive

I know that the package is meant to not use them directly, but sometimes it could be useful to be able to insert nonces and hashes directly instead of relying on the given API methods.
It's actually already doable via `csp()` method, but with a somehow hacky notation:

'\'sha256-K28TraF0hDSDYoxDYfyCCb5cCVAhDUyT0P1E3a+hKyQ=\''
"'sha256-K28TraF0hDSDYoxDYfyCCb5cCVAhDUyT0P1E3a+hKyQ='"
'\'nonce-xxxxxxxx\''
"'nonce-xxxxxxxx'"

I've not tested those on your library directly but I use them in laravel-securityheaders and they work.

My request, if possible, is to add two more "friendly directive":
- one for nonces, which checks if the string start with a "nonce-" and automatically adds the single quotation marks, while preserving the actual nonce value
- and one for hashes, which checks if the string starts with a "sha[acceptableVersionOfSHA]-" and automatically adds the single quotation marks, while preserving the actual encoded hash value.