Remove deprecated functions
Update CHANGELOG
Make `option` take the reader as a parameter (fixes #63)

This commit implements the plan described in the ticket:

- the `reader` modifier is gone, as well as the `optReader` field of
  `OptionFields`
- `option` now takes the reader as argument
- `nullOption` is now deprecated in favour of `option`
- added `strArgument` as a synonym for `argument str`, to mirror `strOption`