Add support for Go 1.11 modules
Add support for Go 1.11 modules

This change adds a `go.mod` file. An empty file is sufficient since this
project has no external dependencies. For people that want to use the static
version of libgit2, this module can be vendored and the following can be added
to their `go.mod` file:

    replace github.com/libgit2/git2go => ./vendor/github.com/libgit2/git2go
libgit2/git2go,pks-t/git2go,clns/git2go,clns/git2go,clns/git2go,libgit2/git2go,pks-t/git2go,libgit2/git2go