Emit stream frame boundaries in NT/NQ output

Follow-up of #3

Add comments in the output NT/NQ stream to indicate frame boundaries. Something like:

```turtle
# Frame 1
<s> <p> <o> .
... more triples ...

# Frame 2
<s> <p> <o> .
... and so on
```

Set it to be emitted by default. Make it possible to disable with both `--out-no-frame-boundaries` and `--out-minimal`. The latter will later be used to disable all such comment-y outputs at once.