feat(cfssljson): Add -output argument to save files in another directory

[eddyzags]

Problem

Currently, cfssljson doesn't provide any way to write the separate key, certificate, csr, and bundle files in a specific folder. This is inconvenient if we want to output the certificates in a different directory.

A workaround would be to use the -stdout argument, which writes every file in the standard output separated by a blank line. Then, redirect the output to a specific folder.
Example:

cfssl gencert -initca ca_csr.json | cfssljson -bare -stdout ca > path/to/folder/certs

But, this approach writes every entity in one file, then requires us to re-process the output of cfssjson to separate the key, certificate, csr, and bundle.

This PR adds an --output argument to the cfssljson tool to place output files into a specific directory.