Expose fs as an option to override the default implementations

test/integration/liquid/fs-option.ts

@@ -0,0 +1,21 @@
+import { expect } from 'chai'
+import Liquid from '../../../src/liquid'
+
+describe('LiquidOptions#fs', function () {
+  let engine: Liquid
+  const fs = {
+    exists: () => Promise.resolve(true),
+    readFile: () => Promise.resolve('test file content'),
+    resolve: () => 'resolved'
+  }
+  beforeEach(function () {
+    engine = new Liquid({
+      root: '/root/',
+      fs
+    })
+  })
+  it('should be used to read templates', function () {
+    return engine.renderFile('files/foo')
+      .then(x => expect(x).to.equal('test file content'))
+  })
+})