Provide simple API for tags

Closes #6

README.md

@@ -125,6 +125,26 @@ A for loop allows you to iterate over an array found by variable lookup.
 {% endif %}
 ```
 
+#### Building custom tags
+
+You can build a custom template tag. There are a couple of APIs to allow
+you to write your own custom tags. The following is the simplest form:
+
+```swift
+template.parser.registerSimpleTag("custom") { context in
+  return .Success("Hello World")
+}
+```
+
+When your tag is used via `{% custom %}` it will execute the registered block
+of code allowing you to modify or retrieve a value from the context. Then
+return either a string rendered in your template, or an error.
+
+If you want to accept arguments or to capture different tokens between two sets
+of template tags. You will need to the `registerTag` API which accepts a
+closure to handle the parsing. You can find examples of the `now`, `if` and
+`for` tags found inside `Node.swift`.
+
 ### Comments
 
 To comment out part of your template you can use the following syntax:

Stencil/Node.swift

@@ -52,6 +52,18 @@ public func renderNodes(nodes:[Node], context:Context) -> Result {
     return .Success(result)
 }
 
+public class SimpleNode : Node {
+    let handler:(Context) -> (Result)
+
+    public init(handler:((Context) -> (Result))) {
+        self.handler = handler
+    }
+
+    public func render(context:Context) -> Result {
+        return handler(context)
+    }
+}
+
 public class TextNode : Node {
     public let text:String
 

Stencil/Parser.swift

@@ -41,6 +41,12 @@ public class TokenParser {
         tags[name] = parser
     }
 
+    public func registerSimpleTag(name:String, handler:((Context) -> (Stencil.Result))) {
+        registerTag(name, parser: { (parser, token) -> TokenParser.Result in
+            return .Success(node:SimpleNode(handler: handler))
+        })
+    }
+
     public func parse() -> Results {
         return parse(nil)
     }

StencilTests/StencilTests.swift

@@ -64,4 +64,16 @@ class StencilTests: XCTestCase {
         let result = template.render()
         XCTAssertEqual(result, Result.Success("Hello World"))
     }
+
+    func testSimpleCustomTag() {
+        let templateString = "{% custom %}"
+        let template = Template(templateString:templateString)
+
+        template.parser.registerSimpleTag("custom") { context in
+            return .Success("Hello World")
+        }
+
+        let result = template.render()
+        XCTAssertEqual(result, Result.Success("Hello World"))
+    }
 }