Allow simple grouping of values with "." in names

[dylex]

That is, the following assignments could be equivalent:

group {
  a = 1
}
group.a = 1

This doesn't seem like it would introduce any incompatibilites (since "." is not allowed in names), but would make the configuration format more flexible. I can work on a patch if there is interest, probably just by parsing idents with "." as an implicit singleton Group (or as many as there are dots).