This introduces CertificateFingerprints.from_string(), which takes in

a string value, normalizes it, and attempts to determine if it

represents a SHA-1 or SHA-256 value. If it's either of these, an

instance of CertificateFingerprints will be created and returned with

the appropriate field filled out. Otherwise, None will be returned.
This is useful for cases where we're working with SCMs that provide a

single fingerprint that may be in either of these formats.

Unit tests pass.