Add overrides option to override handlebars public API functions.

This will let you override handlebars [public API functions](https://github.com/wycats/handlebars.js/blob/7f6ef1dd38794f12aee33c76c04f604a7651810b/lib/handlebars/compiler/javascript-compiler.js#L10) in your brunch-config file.
Update README to include the overrides plugin option.
Merge pull request #21 from elving/master

Add overrides option to override handlebars public API functions.