Add filter for INI sections

[matbu]

Sometimes we don't want to produce a diff for all the ini file, which can be huge with a lot of missing sections.
We need to add a filter which can just produce diff against this specific sections and ignore others.

[matbu]

something such as:
os-diff diff file1 file2 --section-filter default,foo,bar

will only diff against:
[default]
...
[foo]
...
[bar]
...

[davidjpeacock]

I think you did this already @matbu ?