Configuration system

[parafoxia]

Severn should have a customisable config that can be expanded over time.

It should be able to read pyproject.toml files (looking for tool.severn), and potentially a .severn.toml as well. The .severn.toml file should be read first, and the pyproject.toml afterwards.

Acceptance Criteria:

• A single class which serves as the config parser.
• After a successful parse, the class properties should be populated with config values.
• Use sensible defaults for config values that are not specified.

Actual config options TBD at this point, but make the system easily extendable for new config options in the future.

[Jonxslays]

I'll start putting something together for this.