Time-varying DAGs

Would be nice to come up with a parsimonious scheme to declare DAGs with time-varying nodes, where quickdag handles the time-indexing and labeling for the user.

For instance, something like:

``` r
qd_dag(c("L~~k <-> A~~k",
         "L~~k -> Y",
         "A~~k -> Y"),
         k = 3)
```

Where the user will get a diagram with two timepoints representing the proper lags.