allow putting response fields in a gap in the item label

[rubenarslan]

In some cases, it can be prettier to have the response be part of the sentence flow, as in

 I've [never/sometimes/always] wanted to be a rockstar.

We could look for a highlight word like [response] in the item label. Substitution makes sense for types like number, select-family, text-family, but not really for mc etc.

A simpler case would simply be to allow users to specify a unit before/after the number input in a more simple case. Currently this can be achieved by customising the run CSS. Assign the item a class like "times", then add the following to the run CSS.

.item-number.times input[type=number]::after {
content:" times"
 }