I propose to allow the `-` char in allowed label characters (currently a-zA-Z0-9_ https://www.postgresql.org/docs/9.6/static/ltree.html)
The reason is to allow to use more easily base64 ids, (like https://github.com/dylang/shortid, ...), uuid's too in labels `-` is also not used as a query operator as a plus source: https://doxygen.postgresql.org/ltree_8h.html# a1cb5d5dd0c364d720854e8a250967dd1
