Adding gpgid field

[HugoJH]

Added gpgid field so GPG keys can be configured to be used on that repo.

[DrVanScott]

Hi, i like the idea of adding the optional possibility for a gpgid. But I don't agree with the default commit.gpgsign=true.

Linus Torvalds about this topic:

Signing each commit is totally stupid. It just means that you automate it, and you make the signature worth less. It also doesn't add any real value, since the way the git DAG-chain of SHA1's work, you only ever need one signature to make all the commits reachable from that one be effectively covered by that one. So signing each commit is simply missing the point.

So, i would prefer either not setting commit.gpgsign or also make this one configurable (with by default not to change this setting). What do you think?

[DrVanScott]

I will close this PR but open another one based on your changes: #9