Feat: add support for adding useful parts

[aichbauer]

if you have a template that should not be included within the creation of the app with sepp start, but something that is often used.

E.g. in an express application: a model, a controller... the basic code is always the same.

Maybe we can add a new folder inside the sepp folder of a template, called parts. inside this folder we can place useful parts. And create it inside our app with a command like sepp template-name generate useful-part-name