fix: always store value for a=b

[shadowspawn]

This actually has two fixes in just two statements. They overlap, so not separate PR. :-)

The big change is always store the value for --foo=value, even for zero-config. See #24 for discussion to support this. This approach:

• reflects user intent
• preserves information for author curation
• does not introduce a new pattern in the shape of the result

Closes #24.

The small fix is --foo=b=c stores the value b=c (previously was ignoring text from the second equals onwards).

(Code previously proposed and reviewed in #26, so will look familiar logic to some.)

[shadowspawn]

Thanks reviewers. I will merge this weekend if nothing else comes up.