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

1 files changed, 1 insertions, 0 deletions

diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..688b8e3
--- /dev/null
+++ b/go.mod
@@ -0,0 +1 @@
+module github.com/libgit2/git2go