The initca option can be used with an existing private key.

This is useful for generating a new certificate (i.e. with updated
signature scheme) from an existing private key.

This commit also adds the -bare option to cfssljson, which is used for
the responses from the command line that do not use the API response.
Merge pull request #17 from cloudflare/kyle/initca-existing-key

The initca option can be used with an existing private key.