Add a command to download or print a certificate

[maraino]

Description

Add a command to print or download a certificate. Sometimes is useful to get the PEM version of a certificate, we should add a command like

# Just the cert
step certificate download https://smallstep.com > smallstep.pem
# Bundled with intermediate
step certificate download --bundle https://smallstep.com > smallstep.pem

[maraino]

Consider naming the command as step certificate get
We should verify the certificate before downloading it by default, but we should support an --insecure flag to disable that feature.

[tommy-56]

There is a --format pem option to step certificate inspect that implements this behavior.
Example:
step certificate inspect --format pem https://name.com