Add possibility to include template path in script's id tag

Hello!
I thought that it would be really helpfull to make it able to prefix scripts id tag with underscored path. It can be usefull when someone want's to have a more complex `template` folder and file structure and some templates have the same names beeing in different dirs - that creates a conflict. For instance, both `templates/menus/items.hbs` and `templates/categories/items.hbs` generates a script tag with `id="items_template"`.

With `include_path_in_id` set to true, those templates will have the following ids: `menus_items_template` and `categories_items_template`

Of course we're able to overwrite id with `id` option in `template_include_tag` or we can just prefix our templates somehow but i think that sucks a bit and can be messy for more complex template dir structure.