feat(filter): Support filter tag

Tests/StencilTests/FilterTagSpec.swift

@@ -0,0 +1,25 @@
+import Spectre
+import Stencil
+
+
+func testFilterTag() {
+  describe("Filter Tag") {
+    $0.it("allows you to use a filter") {
+      let template = Template(templateString: "{% filter uppercase %}Test{% endfilter %}")
+      let result = try template.render()
+      try expect(result) == "TEST"
+    }
+
+    $0.it("allows you to chain filters") {
+      let template = Template(templateString: "{% filter lowercase|capitalize %}TEST{% endfilter %}")
+      let result = try template.render()
+      try expect(result) == "Test"
+    }
+
+    $0.it("errors without a filter") {
+      let template = Template(templateString: "{% filter %}Test{% endfilter %}")
+      try expect(try template.render()).toThrow()
+    }
+
+  }
+}

Tests/StencilTests/XCTest.swift

@@ -17,6 +17,7 @@ public func stencilTests() {
   testNowNode()
   testInclude()
   testInheritence()
+  testFilterTag()
   testStencil()
 }