Reactive config

[jgraef]

We need config management with the following features:

• [x] parse
• [x] write
• [x] react to file modifications
• [ ] fine-grained reactivity. We want to send signals to sub-systems when some of their configuration values change.
• [ ] transactional, fine-grained modifications (i.e. through UI components).
• [ ] good diagnostics (filename, line, column)
• [ ] generate documented default/example config (optional)
• [x] TOML
• [ ] YAML. then we can use this for #15