Inbox POST from federated servers now working for statuses and follow requests.
Follow request client API added.
Start work on federating outgoing messages.
Other fixes and changes/tidying up.
* add host and protocol options
* some fiddling
* tidying up and comments
* tick off /oauth/token
* tidying a bit
* tidying
* go mod tidy
* allow attaching middleware to server
* add middleware
* more user friendly
* add comments
* comments
* store account + app
* tidying
* lots of restructuring
* lint + tidy