[Feature request] List all defined ip routing tables

I would like to see `ip route show tables` as a command to list all
currently defined tables in the system. I can kind of get hints of
what exists through `ip rule`, but if I delete a rule, the table still
lingers.

I don't mean listing the contents of all tables like `ip route show
table all` does but instead just list the table id, its alias and
maybe an integer of how many entries it contains