feat: Added indent filter (#188)

Tests/StencilTests/FilterSpec.swift

@@ -244,4 +244,30 @@ func testFilter() {
 
   }
 
+
+  describe("indent filter") {
+    $0.it("indents content") {
+      let template = Template(templateString: "{{ value|indent:2 }}")
+      let result = try template.render(Context(dictionary: ["value": "One\nTwo"]))
+      try expect(result) == "One\n  Two"
+    }
+
+    $0.it("can indent with arbitrary character") {
+      let template = Template(templateString: "{{ value|indent:2,\"\t\" }}")
+      let result = try template.render(Context(dictionary: ["value": "One\nTwo"]))
+      try expect(result) == "One\n\t\tTwo"
+    }
+
+    $0.it("can indent first line") {
+      let template = Template(templateString: "{{ value|indent:2,\" \",true }}")
+      let result = try template.render(Context(dictionary: ["value": "One\nTwo"]))
+      try expect(result) == "  One\n  Two"
+    }
+
+    $0.it("does not indent empty lines") {
+      let template = Template(templateString: "{{ value|indent }}")
+      let result = try template.render(Context(dictionary: ["value": "One\n\n\nTwo\n\n"]))
+      try expect(result) == "One\n\n\n    Two\n\n"
+    }
+  }
 }