Support PKCS8 for RSA keys

[mhoshi-vm]

My software requires PKCS8 format rsa keys, but it seems this tool only can output PKCS1 format

Want a spec to choose the format

[rohitagg2020]

Hey @mhoshi-vm We had a discussion about this, and we are okay with the above enhancement.
Would you like to raise a PR making the change? We will prioritise a review.