feat(cmd): add --rsa-bits option

It indicates the bit size used for RSA keypair
fix(pkix): CommonName and Verify function doesn't match

CommonName indicates host IP, while OrganizationalUnit presents host name
after e05dfb61.
It is different from original design, and all related test data
should be updated.
chore(cmd): Rename rsa-bits to key-bits
Merge pull request #12 from unihorn/35

add --rsa-bits option