Allow specifying multiple static assets

[wch]

Previously, in calls to App(), the static_assets could only be a single directory, as in:

App(..., static_assets=Path(__file_).parent / "www")

This means that the www/ subdirectory on disk would be mounted at /.

This PR makes it possible to pass in multiple items, mounted at multiple locations. The API looks like this:

App(
  static_assets= {
    "lib" : Path(__file__).parent / "lib",
    "image.jpg": Path(__file__).parent / "image.jpg"
  }
)

This can be done more concisely by using a dict comprehension, as in:

_static_assets = ["lib", "image.jpg"]
_static_assets = {"/" + sa: Path(__file__).parent / sa for sa in _static_assets}

Unfortunately, I don't think there's a way to avoid passing in __file__ somehow, at least not without doing something really hacky.