Add Rugged::Config#add

[codenamev]

This method allows you to set multiple values for the same key in a
particular config, similar to git config --add "include.path" some/path.

For example, I have a git config with an existing include.path value:

$ cat ~/src/opensource/rugged/.git/config
[core]
        repositoryformatversion = 0
        filemode = true
        bare = false
        logallrefupdates = true
        ignorecase = true
        precomposeunicode = true
[remote "origin"]
        url = git@github.com:codenamev/rugged.git
        fetch = +refs/heads/*:refs/remotes/origin/*
[branch "master"]
        remote = origin
        merge = refs/heads/master
[submodule "vendor/libgit2"]
        active = true
        url = https://github.com/libgit2/libgit2.git
[include]
        path = /Users/codenamev/.gitconfig.reflow

With Rugged::Config#add I can set another value for include.path
without overwriting the existing value:

ruby -I lib -rrugged -e'\
config = Rugged::Repository.new("/Users/codenamev/src/opensource/rugged").config;
config.add("include.path", "/Users/codenamev/.gitconfig.extras");
p config.get_all("include.path")'
["/Users/codenamev/.gitconfig.reflow", "/Users/codenamev/.gitconfig.extras"]

References #725

[codenamev]

I took this snippet from the libgit2 source. I'm not entirely sure this is the best way to do this conversion, but I figured I'd at least mimic the behavior. There are some performance enhancements related to windows systems for snprintf that were not ported over (because I have no idea how that works).