Refactor Hugo version

Put version handling into the helpers package so it can be used by many,
and split version and suffix to make it possible to calculate the next Hugo version.

helpers/hugo_test.go

@@ -0,0 +1,11 @@
+package helpers
+
+import (
+	"github.com/stretchr/testify/assert"
+	"testing"
+)
+
+func TestHugoVersion(t *testing.T) {
+	assert.Equal(t, "0.15-DEV", hugoVersion(0.15, "-DEV"))
+	assert.Equal(t, "0.17", hugoVersionNoSuffix(0.16+0.01))
+}