Add feature to vendor openssl

[tshepang]

To help build more portable executables, it would be nice to have an option to build against a vendored openssl version.

At the moment, I have to do the following myself, even though openssl is a transitive dependency:

# This is a hack, to force openssl to be built statically.
# It's a dependency of crypto-hash.
[dependencies.openssl]
version = "0.10"
features = ["vendored"]

[malept]

Can you explain why this is a hack?

[tshepang]

Am controlling how a transitive dep is built, and that makes it look like I depend directly on it. Would be better if there was a cargo feature (in crypto-hash) to choose.

[malept]

I'm happy to take a pull request for this. Please make sure that it gets documented in the README, too.