Use Click to parse configuration

[lemon24]

We currently have a hand-rolled configuration loader added in #177 that accomplishes two things:

• it can be used from the CLI and from the web app
• it allows merging options from multiple contexts (default, CLI, web app), including list options (plugins)

However, it also has a bunch of issues:

• the config format is bad (it targets arguments to make_reader(), with CLI defaults tacked on; also, the contexts thing is overkill)
• overriding make_reader() options from the CLI is cumbersome
• it's orthogonal to plugin loading (and for CLI / web app plugins, it doesn't help at all)
• you can't set arbitrary options via environment variables (now we support 5 hardcoded variables in 2 separate places)

Instead, we could use Click to parse configuration for everyone, since it already mostly does most of the work:

• setting default_map from a config file can provide defaults for all options of all commands
  • => single configuration format, that of the CLI; the web app can use the serve options
  • we can still merge defaults and command-line values using option callbacks, where it makes sense
  • overriding options from the CLI / environment variables comes out of the box
  • we can load plugins (e.g. @click.option(..., type=pkgutil.resolve_name))
• it's possible to get Click to load the config and environment variables without invoking the commands
  • => free config parser (including for the web app)
  • perhaps we can also use TOML instead (tomllib is in stdlib)

The downside is that we can only set the make_reader() options that are exposed in the CLI (great incentive for parity :P).

Prototype here: LINK. (https://github.com/phha/click_config_file does kinda the same thing, but only the first part.)

Related issues:

• CLI options must be passed all the time #177
• Consider using Confuse for configuration #208 (superseded)
• Minimal API to enable/disable built-in plugins #229
• Plug-in API #80

[lemon24]

Feature is mostly done; remaining work:

• test_config_option
• changelog
• docs