require() should handle paths as relative (like nodeJS) (#443)

* Integrate @nlowe's documentation and tests
* Use filepath.Clean() instead of filepath.Abs()
* Use a stackless implementation

docs/_functions/global/require.md

@@ -0,0 +1,52 @@
+---
+name: require
+parameters:
+  - path
+---
+
+`require(...)` behaves similarly to its equivalent in node.js. You can use it
+to split your configuration across multiple files. If the path starts with a
+`.`, it is calculated relative to the current file. For example:
+
+{% include startExample.html %}
+{% highlight js %}
+
+// dnsconfig.js
+require('kubernetes/clusters.js');
+
+D("mydomain.net", REG, PROVIDER, 
+    IncludeKubernetes()
+);
+
+{%endhighlight%}
+
+{% highlight js %}
+
+// kubernetes/clusters.js
+require('./clusters/prod.js');
+require('./clusters/dev.js');
+
+function IncludeKubernetes() {
+    return [includeK8Sprod(), includeK8Sdev()];
+}
+
+{%endhighlight%}
+
+{% highlight js %}
+
+// kubernetes/clusters/prod.js
+function includeK8Sprod() {
+    return [ /* ... */ ];
+}
+
+{%endhighlight%}
+
+{% highlight js %}
+
+// kubernetes/clusters/dev.js
+function includeK8Sdev() {
+    return [ /* ... */ ];
+}
+
+{%endhighlight%}
+{% include endExample.html %}