Add layer to handle alpha_ids as the primitive node reference

[jrgant]

[jrgant]

An obvious issue suggested by the need for verbose to be the default in qd_dag(), which leaves alpha-to-numeric label matching up to the user to verify, is that quickdag should be doing this sort of QC under-the-hood.

Allowing users to input either numeric or alpha IDs suggests they should also be able to refer to either when selecting nodes to style or reposition, without wrestling with DiagrammeR's selection functions. Provide wrappers/functions to make graph manipulation easier.

[jrgant]

This layer has been added.