Add utility for loading a secret key from a defined format

The secret key for use with e.g. secretbox should be 32 random bytes. A common mistake is to use only bytes that can be easily typed on a keyboard, like "secret password change me [padded to 32 bytes]" or E%m"lBVK=9&m6GRkm9n'c17C6Csd9?0C.

Part of the reason people do this is that E%m"lBVK=9&m6GRkm9n'c17C6Csd?0C is easier to store in e.g. a YAML file or environment variable than b'\xbf\x9e\x0e\xe6J\xe0T,\xc6\xa5\x81i\xc9\xf2\xb1#?\xf1\xa8\x96Q\xc9\x84\xe9\x8f\xe5\xec\x921\x06\xb2\xef'.

It might be good to define a standardized format in nacl.utils for loading printable, config-safe keys, into 32 bytes of random. One would be to load 64 hex bytes, like a20fbda2e9143075aa2a919c941eaad86652b17eacc4bc816fe399ce2b78c11d, into 32 random bytes.

Then you would put SECRET_KEY=a20fbda2e9143... in the environment and write:

key = nacl.utils.load('a20fbda2e9143075aa2a919c941eaad86652b17eacc4bc816fe399ce2b78c11d')
# or
key = nacl.utils.load(os.environ['SECRET_KEY'])

or load_32 etc. I maintain a Go nacl client and this is the strategy I use for letting users load keys. https://godoc.org/github.com/kevinburke/nacl#Load

Alternatively, you could have load_from_password('hunter2'), which could derive 32 bytes of random from a string using a suitable key derivation function like scrypt.

Start by reading the existing nacl.utils API and the referenced Go nacl Load implementation. Resolve whether the utility should standardize hexadecimal loading, password-based derivation, or both; done means the format and behavior are agreed and the selected utility is implemented and documented.