Starlette-context

* add `RawContextMiddleware` for `Streaming` and `File` responses
* add flake8, isort, mypy
* small refactor of the base plugin, moved directories and removed one redundant method (potentially breaking changes)

- add docs on read the docs
- fix bug with `force_new_uuid=True` returning the same uuid constantly
- due to ^ a lot of tests had to be refactored as well

- for correlation id and request id plugins, add support for enforcing the generation of a new value
- for ^ plugins add support for validating uuid. It's a default behavior so will break things for people who don't use uuid4 there. If you don't want this validation, you need to pass `validate=False` to the plugin
- thanks to VukW you can now check if context is available

- dropped `with_plugins` from the middleware as Starlette has it's own way of doing this
- due to ^ this change some tests are simplified
- if context is not available no LookupError will be raised, instead there will be RuntimeError, because this error might mean one of two things: user either didn't use ContextMiddleware or is trying to access `context` object outside of request-response cycle

- changed parent of context object. More or less the API is the same but due to this change the implementation itself is way more simple and now it's possible to use `.items()` or `keys()` like in a normal dict, out of the box. Still, unpacking `**kwargs` is not supported and I don't think it ever will be. I tried to inherit from the builtin dict but nothing good came out of this. Now you access context as dict using `context.data`, not `context.dict()`
- there was an issue related to not having awaitable plugins. Now both middleware and plugins are fully async compatible. It's a breaking change as it forces to use await, hence new minor version

breaking changes

one middleware, one context, multiple plugins for middleware
very easy testing and writing custom plugins