Type-config

Fixed
- Falsy values are now considered when merging (a `False` will be considered a set value, not something left un-set).
This was a problem when using `argparse`'s '`action="store_false"`.
It also implies that when using `store_false` or `store_true` and you don't desire a default False (which could overwrite the config file), you also need to specify `default=None`.

Added
- It's now possible to use multi-line strings for `help` and `important_help` messages.
- Changelog.md has been added to the project.

Changed
- Changed `add_option`'s `constraints` parameter to `important_help`.