Make yas-describe-tables easier to use from lisp

Relating to #443.

I'm wondering about the namehash tables: how come it shows the names inside lists? Surely a key can only map to a single name?

* yasnippet.el (yas-describe-table-by-namehash): new function.
(yas-describe-tables): remove query for by-namehash variant, replace
query for non-active tables with prefix arg.