load.project issues

- [x] Use `$` to access config options
- [ ] Redundant code for checking for missing entries in `config`: Extract to function
- [ ] Options that allow `on`/`off` should be converted to `logical`
- [ ] Use `pattern` parameter in `dir()`
- [ ] Create necessary directories (cf. #35)
- [ ] Redundant code for loading from cache
- [ ] Extract functions, one per step
- [ ] Allow overriding (some of the) configuration options by (a) new parameter(s)