Support for milligram's flexbox-based columns

milligram's flexbox-based column selectors and rules have their own section in sscaffold and are largely unchanged, with one important exception: milligram's .container selector has been replaced by the main article, main section, main section > *, and main article > * selectors later in the file, in the "theme" section.

Rows and columns need to be inside any block-level element inside a section or article element inside the main element to prevent conflicts between milligram's .row rules and other selectors:

                <div class="row">
                    <div class="column">
                        <!-- ... -->

You can see a more complete example on, which is also a handcrafted, not-minified demo page.

There is a more complete explanation of this change on the differences from milligram page.