Support width for checkbox and radio

[mootari]

Checkbox and Radio are currently the only widgets without support for a custom width.

In the context of these widgets, width would define the max width at which input/label pairs wrap onto the next line. This width is currently hardcoded to 640px.

[CobusT]

Hope you don't mind that I transfer this to the Inputs repo so that it can be worked there...

[Fil]

Want it for Inputs.toggle, too—the label is often long and gets wrapped.