…nd multiple occurrences

When using number_of_values() or value_names() you no longer have to set
.multiple(true) unless you want the argument to be allowed multiple
times (i.e. *not* the value, but the argument itself). This means
without multiple(true) set (and assuming 2 values) -o val1 val2 is only
allowed one time, but with multiple(true) set, -o val1 val2 -o val3 val4
is allowed.

Closes #99