update help output to reflect that some options cannot be set in the config file

[globau]

wintersmith build -h lists options, including --clean.
it also states all options can also be set in the config file.

turns out that not all options can be set; for example clean is not honoured (see #149).

the wording of the help text should be updated to reflect that some, but not all, options can be set. ideally those that are cli-only should have an indicator.