add route_static_path flask util

this util is used for have a shorter route to a static path

e.g.
I have static path of 'myapp/static/some_static_project/' which I
want to expose in the route '/flowers, use:

route_static_path(app, '/flowers', 'some_static_project', 'flowers')

then, if I can acess this file
'myapp/static/some_static_project/index.html' by going the url -
'flowers/index.html'.

This can be useful if you want to bring in a git submodule of a static frontend
project and expose it through a nice url.