Support stretch alignment

[peterstuart]

This tries to mimic the behavior of the stretch alignment in CSS flexbox. In an example where the main axis is vertical:

• stretch alignment
  • widgets with no set width will be drawn will the full width of their parent
  • widgets with a set width will keep that width and be aligned as if the alignment was "start"
• other alignments
  • widgets with no set width will have a width of 0 (so consume vertical space, but not be visible)

Fixes #9.

Testing

Remove the width from yellow_box and/or magenta_box, and see how that interacts with the different alignments you can set on container (try the new Stretch alignment).

[peterstuart]

@helixbass Yeah, I'll do that next. That seems like it should be pretty easy to do.