Adding subjectAlternativeNames support

[Muffinman]

As discussed in #22, this PR adds subjectAlternativeNames support.

The following is currently untested, but just wanted to check direction before I go any further.

[Daanra]

Thanks for the effort!

I think it can be done without breaking changes by adding the SAN functionality to the PendingCertificate class.

For example:

LetsEncrypt::certificate('mydomain.com')
        ->subjectAlternativeNames(['mydomain2.com'])
        ->create(); 

[Muffinman]

Hi @Daanra,

I had just assumed those create() and createNow() methods on the Facade would have to be updated. If doing as you suggest this makes for a far simpler PR.

[Daanra]

@Muffinman Thanks, looks great!