Pass extra settings using ** in application.wsgi
Simplify & improve settings handling/parsing

- Remove settings parsing logic from `Application.__init__()`.
- Remove `parse_settings` arg from `Application`.
- Remove `Application.parse_settings()` and
  `Application.parse_settings_file()`.
- Remove `settings.parse_settings()` and
  `settings.parse_settings_file()`.
- Add a new function, `settings.make_app_settings()`, to handle
  settings parsing.
- Everything that was removed was subsumed by `make_app_settings()`.
  When apps need special settings handling/parsing, they should call
  `make_app_settings()` directly and pass the result to the
  `Application` constructor.
- Make `make_app_settings()` part of the application API (i.e., update
  docs to indicate this).