Fix examples in readme that use javascript syntax instead of Lua.
Pass un-rendered text to functions in keys.
Previously the section text passed to functions was already
rendered however per readme it should be un-rendered so that the
function can operate on the raw text and pass the result to the
"special render function" that it is passed to render the
result. To pass the un-rendered text this change stores the start
and end index of each token in the original template. With this
information the original un-rendered text for a section can be
extracted from the original template and passed to the key
function. This closely mirrors the implementation in mustache.js.

Don't pass self to section key functions. Makes code match readme
and makes it closer to mustache.js.
Merge pull request #14 from jhandley/master

Fix function example in readme