Options to specify acme-server and turn off certificate check of the imap server

[grindsa]

Hi,

I am developing an acme-proxy and would like to implement support of RFC8223. To facilitate testing, I plan to use your client. However, I require the following features to proceed:

• An option to specify a custom ACME server (other than acme-castle).
• The ability to ignore SSL validation errors for both the ACME server and the IMAP server.

These options are already available in Certbot (--server and --no-verify-ssl), and for consistency, I have adopted the same naming convention in your implementation.

I think these parameters could be useful for others as well. Hence i am sending this PR.

Thank you for your work on this. I hope my contribution can be considered.

/G.

[polhenarejos]

Please do not forget to update README accordingly with new parameters.

[grindsa]

done