Allow a custom prop for stack form input column widths

[steverydz]

The column width for labels in the Input component in a stacked form is 4. On Snapcraft listing forms we use a column width of 2 meaning this component won't work for our layout. It would be good if there were custom props to allow changing the default.

[bartaz]

Sounds like a reasonable request. Although we would need to do it for Input and Fields components (as one wraps around another), and there probably needs to be 2 props (one for label and one for input widths). So I'm not sure how complicated it will end up.