Allow escaping of characters in the delimiter

[Laschoking]

Currently the delimiter must be a single character.
Maybe the escaping of characters should be allowed to have quotes as separator.
@export Test :- dsv{delimiter="\""}.

This would also allow an alternative formulation for tsv files with @export Test :- dsv{delimiter="\t"}. , that can be beneficial when translating Souffle into Nemo, and vice versa.