Support for multiple subdomains

[ndbroadbent]

Instead of issuing multiple certificates for each subdomain, it would be useful if we could create a single certificate that includes multiple subdomains. Not sure about the best way to add support for this

[elct9620]

My colleague works on another project is add this feature, you can reference his fork (https://github.com/lctseng/rails-letsencrypt/commits/master)

This feature I will consider it, but too large certificate bundle may slow down client and server because it needs more CPU to calculate it. Maybe another way is using the wildcard certificate, you can subscribe #3 issue after I can add this feature.