cfssl certinfo should be pipeable into cfssl genkey -

`cfssl certinfo` should be pipeable into `cfssl genkey -` to generate a new csr and key based off of a preexisting cert, csr or domain.

i.e.
```
cfssl certinfo -domain www.cloudflare.com | cfssl genkey -
```
This would be useful for renewals.

Currently, I end up with pretty much none of the data translating into the new csr/key
* No subject
* No san
* No matching key algorithm

1) `cfssl certinfo` doesn't provide full key details.

i.e. no `"key": { "algo": "rsa", "size": 2048 }` or equivalent

2) `cfssl certinfo` and `cfssl genkey` don't represent data the same way, nor does `cfssl genkey` seem to alias or understand `cfssl certinfo` data.

i.e.
`cfssl certinfo` .sans vs `cfssl genkey -` .hosts
`cfssl certinfo` .subject (and sub-key names) vs `cfssl genkey` .names (and sub-key names)