Add support for passing md5/sha256 parameter in Gravatar provider

[pkcs8]

At the moment, the Gravatar provider only supports passing email address parameter. For public websites, this may not be the best approach since it would expose user's email addresses. Since Gravatar API supports requesting avatar images by md5/sha256 only, there's no need for unavatar to compute the md5 hash. The user can just pass the hash to unavatar and it should work.

For backward compatibility reasons, perhaps both email address and hash may be supported.

[Kikobeats]

Interesting.

Unavatar translates the input (email, username, domain) into a URL.

The cool thing could be supporting MD5 as input for all the providers, not just Gravatar (otherwise, you can query against Gravatar directly, so what's the Unavatar benefit here?)

Related: #19