Generate gpg.conf from m4 template on make call

We have to do this because gpg.conf doesn't understand tilde or
environment variable expansion in the configuration file, and the only
reliable way to make the ca-cert-file option work between different
implementations of gpg(1) and its cURL link is to explicitly specify the
path to the CA file.

This is probably a better approach than installing the thing as a
trusted system CA anyway, which requires root privileges that I don't
really want to assume anyone installing this has.

I'm also including the CA, CRL, and .pem for the SKS keyservers in this
commit. This seems a lesser evil than trying to pull them with cURL or
wget at make(1) time.
Pull Vim plugin submodules in separate target

Tidier. Still don't like that I'm doing something network-related in a
Makefile, but it's better than what I had.