Allow to specify a destination file to be able to group routes

[php-coder]

At this moment all routes are placed into a single file called routes.{lang}. It should be possible to group routes by files. I suggest to add a new parameter with a file name:

- path: /v1/categories/:categoryId
 # NOTE: there is no extension to not depend on an implementation
 # This route will be placed into `categories_routes.py` file (for Python) and included into `app.py`
 file: categories_routes
 get:
 query: SELECT id, name, name_ru, slug FROM categories WHERE id = :p.categoryId

[php-coder]

Should be done after #28