load.project issues

- [x] Use `$` to access config options
- [x] Use `$` to access `my.project.info`
- [x] Get default configuration as list
- [x] Redundant code for checking for missing entries in `config`: Extract to function
- [x] Options that allow `on`/`off` should be converted to `logical`
- [x] Use `pattern` parameter in `dir()`
- [x] Vectorize skipping of deprecated files
- [x] Make use of Boolean configuration options
- [x] Use `require.package`
- [x] Create necessary directories (cf. #35)
- [x] warn instead of stopping if configuration file is missing
- [x] Redundant code for loading from cache
- [x] ~~Extract functions, one per step~~
- [x] allow overriding (some of the) configuration options by (a) new parameter(s)
- [x] warn on extra entries in the configuration
- [x] ~~avoid duplication of `global.dcf`~~ (#79)