Column macro: Styling improvements

[snazare]

Is your feature request related to a problem? Please describe or add steps to reproduce your use case.

1. Columns should take the full width when the width is not specified. It's not currently the case, and in the html generated by the column macro, there's an uninitialised velocity variable that appears.

2. CSS improvements: the section macro has a display:flex styling, but then the columns inside them are display:table-cell, and there's no parent element with a display:table-row and a display:table CSS. This is mixing two very different and incompatible ways of doing columns in CSS