MinWidth, MaxWidth

Hey!

This is great. A cooperation with https://github.com/jroimartin/gocui would have been cool, if possible.

Anyway, I was wondering if some kind of "MinWidth"/"MaxWidth" could be specified. This would have been very handy. With these, it would try to keep these values as much as it could, when layouting. For example, now I have a list column to the left that I don't need to expand in width when resizing. And, It gets a bit too small to fit the text when going down in size (I use NewCol(2), NewCol(10)), so having these around would make it possible for me to specify a size range it could be within, while the "column" to the right of it expanded as appropriate.

Thanks!

Start with the column construction and layout behavior described around NewCol(2) and NewCol(10), focusing on how widths change during resizing. Done means columns can accept MinWidth and MaxWidth values and keep their widths within that range as the layout adjusts.

Written by the indexing model from the issue text.