Add support for custom handlebars setup

[papb]

My idea is related to #89, but instead of adding the specific replace helper, I think an extra option called --handlebars-setup would be great. Example: auto-changelog --handlebars-setup my-setup.js and then on my-setup.js file:

module.exports = function(Handlebars) {
    // Here I can call whatever Handlebars function I wish, for example I can define a custom helper
    Handlebars.registerHelper('custom', function (object) {
        // ...
    });
}