Add support for column layout

[muZk]

As you can see in react-photo-gallery there are 2 layouts: row and column.

In this library, we have both algorithms but the createCollage function is always using the row layout one (here).

To close this issue, a new option to createCollage should be added (layout = 'row'). When specifying row, do the same it is currently doing. For layout = 'column', use computeColumnLayout from src/layouts/columns.js similar to https://github.com/neptunian/react-photo-gallery/blob/master/src/Gallery.js#L76

[giubatt]

I'll work on this

[muZk]

thanks!