config: warn if unexpected settings exist

[Exirel]

Description

Fix #1570

To implement this, I added new methods and changed the bot's setup() method:

• sopel.config.Config.get_defined_sections() retrieves all defined sections, so it's easier to know which sections of the config file are defined in order to check them for unexpected settings
• sopel.bot.Sopel.setup() now calls a new method called post_setup(), to start the scheduler and perform post-setup checks

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[dgw]

This looks pretty straightforward. 🚀 My suggested changes are all in documentation.

[dgw]

@Exirel Go ahead and squash, then we'll let the final history finish checks before I approve it… at the rate Travis is running jobs, maybe tomorrow. 😝

[dgw]

🥳