Add named variables

[lucapette]

Right now, we support only variables in the form:

select * from answers where last_update > {date}

Which prevents users from using the same kind of variable multiple times in the same query. We can solve it adding some syntax that optionally names variables:

select * from answers where last_update > {date:start} and last_update < {date:end}

The variable name would serve as a label as well in the UI.