[Feat] r.univar: add format=shell and CSV

[petrasovaa]

Is your feature request related to a problem? Please describe. To standardize format options, r.univar should have format=shell making -g obsolete and format=csv making -t obsolete.

Option separator decided the delimiter for the current table format, but in the future is would influence the csv format. So for now we could do it the same way v.db.select has it - the option has no default, so if no separator is specified -t flag will use the current default (pipe) and the CSV format will use comma. This way it will be backwards compatible.