Configuration for user-defined auto-require in test files

This is a feature request to allow a user to configure additional namespaces to be automatically required into test namespaces.

I use [matcher-combinators](https://github.com/nubank/matcher-combinators) in a lot of tests, and being able to have that `:require` inserted by default would remove a constant task I have to do. I imagine other folks regularly use a particular testing lib, so I suspect there's wide appeal.

Can I add support for iterating over a user-configured list to https://github.com/clojure-emacs/clj-refactor.el/blob/master/clj-refactor.el#L1131 to allow that?

It should be straightforward to create a list of strings of the same format as `cljr-clojure-test-declaration` et al and insert those one by one to the ns declaration.