add two Interface Functions for expansion feature

add `onIncludeTpl` and `onParseMethod` Interface Functions

1. onIncludeTpl: Use it when `parse` meet `include`
2. onParseMethod: Expand methods by using the string between "<%" and "%>"

For Example:
https://github.com/Bacra/node-d2server/blob/ejs-push-example/lib/module/parseHTML/ejs.extend.js

PS: I'm so sorry. [I pull requests before](https://github.com/visionmedia/ejs/pull/125), but the branch is change. Now the two branches are independent.