Feature Request: Allow setting duplicated properties

Hello thank you for this great library, I'm currently working on a project where I need to add duplicate properties: ```ini [foo] bar=1 bar=2 ``` I see that there is a `get_all()` for property retrieval: ```rust ini.section("foo").unwrap() .get_all("bar"); ``` which would return all `bar` entries. When I now try to use the setter API to add entries, it would always overwrite the entry, which is clearly documented, thanks for that. So I have the proposal to introduce a new API that works like `set` but allows duplicate entries: ```rust ini .with_section("foo") .add("bar", "1") .add("bar", "2"); ``` which would lead to my first listing.