N-Columns dynamic layout

Widescreen monitors are becoming popular (mine is a Dell U4025QW, which is 21:9 ;-) but many a laptop user also spends a lot of time with their screen split vertically 50:50 or maybe 33:33:33.

What would thus be really great is a dynamic "N-Columns" mode, that dynamically splits the screen in equal vertical strips based on the number of windows that are being tiled; and re-sizes them when more are added to/removed from the set.

I appreciate this isn't exactly a shoe-in for the way TilingShell works on the basis of static tiles configuration but I believe this would be very useful from a productivity standpoint (and possibly the only "layout" that many a user could rely on by default).

This is actually the one of the features I'm missing from my previous tiling extension ([Tiling Assistant](https://github.com/Leleat/Tiling-Assistant)), which also happens to be the one Ubuntu currently ships by default (rebranded as "Ubuntu Tiling Assistant"). The other Tiling Assistant feature I'm missing is the ability to assign keyboard shortcuts to activate specific layouts.

I hope I'm not duplicating an existing feature request... Thank you for this awesome extension!