Allow overriding repo config from ENV

[nshp]

Allows e.g.:

ALIEN_OPENSSL_REPO_FTP_HOST=example.com cpanm Alien::OpenSSL

[wbraswell]

Seems legit?

[plicease]

Going to do some testing, but I think I largely approve. I appreciate the documentation on the environment variables, which are also buried in the documentation here:

https://metacpan.org/pod/distribution/Alien-Base/lib/Alien/Base/ModuleBuild/API.pod#PACKAGE-AND-ENVIRONMENT-VARIABLES

[plicease]

I did some testing with my own Alien::FFI, and I vote for inclusion. I will merge this Thursday if there aren't any objections before then.

[plicease]

Rebased and merged. I am going to send this to CPAN now.