Updating layout helpers
Updating layout helpers

Adding a `mode` to layout helpers to allow prepending, appending
or replacing the blocks of content.

doowb/handlebars-helpers,h4/handlebars-helpers,assemble/handlebars-helpers,ColinEberhardt/handlebars-helpers,mciastek/handlebars-helpers,JohnCashmore/handlebars-helpers,helpers/handlebars-helpers,bdoooh/handlebars-helpers,ninjasun/handlebars-helpers,doowb/handlebars-helpers,cgvarela/handlebars-helpers,weekeight/handlebars-helpers,helpers/handlebars-helpers