Add SectionOptions method to return options only in a section.

Here is a scenario to use SectionOptions.

I have several features in my app. Each feature can have a name like "feature1". With SectionOptions, I can write config file as following.

    [features]
    feature1 = on
    feature2 = off
    feature3 = on
    # and more...

In my code, I can just enumerate section "features" to find out which feature should be turned on, instead of enumerate all possible features and see whether config file contains the feature name.