add export-pfx command

[wangtiga]

add export-pfx command, reference by https://github.com/square/certstrap/pull/54

PKCS Format:

If you'd like to convert your certificate and key to PKCS12 format,

$ ./certstrap export-pfx Alice --chain "CertAuth"
Created out/Alice.pfx from out/Alice.crt and out/Alice.key with chain [out/CertAuth.crt]

Alice.key and Alice.crt make up the leaf private key and certificate pair of your choosing (generated by a sign command), with CertAuth.crt being the certificate authority certificate that was used to sign it. The output PKCS12 file is Alice.pfx

or simply run openssl:

$ openssl pkcs12 -export -out out/Alice.pfx -inkey out/Alice.key -in out/Alice.crt -certfile out/CertAuth.crt

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