feat: relativeReference for render/include/layout, #395

- `relativeReference` is enabled by default, set to `false` to disable
- Referenced files are still constrained within root/partias/layouts
- fix: relative filenames are not constrained (which allows arbitrary filesystem read)

Example Usage:

{% render "../foo/bar.html" %}

Note:

../foo/bar.html' should also be within `partials` (or `root` if `partials` not set)

test/unit/fs/node.ts

@@ -5,7 +5,7 @@ import * as chaiAsPromised from 'chai-as-promised'
 
 use(chaiAsPromised)
 
-describe('fs', function () {
+describe('fs/node', function () {
   describe('.resolve()', function () {
     it('should resolve based on root', async function () {
       const filepath = fs.resolve('/foo', 'bar.html', '.liquid')