Declarative config.

[poljar]

The Matrix plugin has macros to define a config in a declarative way, once that is tested out a bit better we'll likely want to import this here.

The macros currently can't take set a callback for the config option, support for this should probably be added before the macros are moved here.

The macros can be found at: https://github.com/poljar/weechat-matrix-rs/blob/master/src/config_macros.rs#L200

[poljar]

This has been imported but it requires nightly, it's behind a config-macro feature flag.

It's still missing a way to define change callbacks for the options.

[poljar]

It doesn't require nightly as of Rust 1.45. The macros now generate docstrings for the section and option getters.

Callbacks for the options are still missing.