Support file-based configuration

[MaddyGuthridge]

It'd be excellent if all of Granian's configuration options could be stored in a file (eg JSON, YAML or preferably TOML). Generally, I much prefer file-based configuration as it allows for documentation of the settings chosen for a particular project within the settings themselves.

[zhu0629]

It's great to be able to configure granian through files like Gunicorn

[beaugunderson]

Agreed, we're migrating from gunicorn and the Python-based gunicorn configuration lets us configure our startup hooks in that file... I don't see any way to do that with Granian short of creating our own Granian instance.