Allowing dash character in LTREE

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