Implement mustache templates rendering

cmd/rednerd/engine.go

cmd/rednerd/serve.go

engines/mustache/engine.go

go.mod

go.sum

models/dataset.go

models/document_extra.go

models/record.go

models/template_extra.go

rendering/template_registry.go

restapi/configure_rednerd.go

restapi/embedded_spec.go

restapi/handlers/render.go

swagger.yaml