Added usage of --challenge-alias, possibility of different DNA modes per domain

[hutmitfrosch]

Since I needed to use acme.sh's --challenge-alias, I forked and wrote it in.
So I wanna participate also here and give my changes back to you, if you are interessted :-)

Allows for something like:

LETSENCRYPT_HOST=foo.domain.example,bar.other-domain.example
VIRTUAL_HOST=foo.domain.example,bar.other-domain.example
LETSENCRYPT_DNS_CHALLENGE_ALIAS=foo.validation-domain.example,bar.other-validation-domain.example
LETSENCRYPT_DNS_MODE=dns_inwx,dns_dynv6
LETSENCRYPT_DNS_MODE_SETTINGS=export INWX_User="my_user" INWX_Password="----" INWX_Shared_Secret="........." DYNV6_TOKEN="????"

• If only one DNS Mode is given, it is used for all domains in the cert.
• If only one challenge alias is given, it is used for all domains in the cert.
• Mode config simply takes all exports for all used APIs.

Can be mixed with un-aliased auth - use "no" instead of alias domain at the according place(s) in the challange alias list like:

LETSENCRYPT_HOST=foo.domain.example,bar.other-domain.example,test.third-domain.example
VIRTUAL_HOST=foo.domain.example,bar.other-domain.example,test.third-domain.example
LETSENCRYPT_DNS_CHALLENGE_ALIAS=foo.validation-domain.example,no,test.some-validation-domain.example
LETSENCRYPT_DNS_MODE=dns_inwx,dns_dynv6,dns_inwx
LETSENCRYPT_DNS_MODE_SETTINGS=export INWX_User="my_user" INWX_Password="----" INWX_Shared_Secret="........." DYNV6_TOKEN="????"

I even tried implementing --domain-alias, but acme.sh's docu sadly is a little bit ambiguous how to mix this with the other two modes, does not support "no" for that parameter and hence filtering for that placeholder was difficult - so I removed those trials again.

I tested for both modes, single cert per domain and SAN.
For single cert per domain, the bullet points above are regarded by filtering out "no" and also applying only one given mode and / or alias for all created certs (see sources).

I am sorry for having no idea how to implement tests fitting your existing ones, so I have to leave that part to someone else.
Might be good to test the whole thing more intensively, seems to fit my use case / needs, so at some point I stoped looking for other edge-cases.

If I find some other bugs, I can do a new pull request, If you like.