Input element configuration

[georgefst]

We should allow some styling options. Currently these can only be plain white boxes.

[georgefst]

It would also be nice to make the step size configurable for number inputs. We set this to "any" in c6ebd4c.

[georgefst]

We may want maxlength as well (#39). And presumably minlength.

[georgefst]

It's worth saying, this is all trivial to add. But we should try to work out everything we want first, rather than making multiple breaking changes.

[georgefst]

See d5c80dd.

So I think we're back to just wanting styling options.