Context classloader support in evaluation.

Added an optional slot in interruptible-eval middleware for a context
classloader, which will be set against the thread during the evaluation
of the code block.

nREPL clients (at least nrepl.el) uses clojure.java.io/resource to
identify the source-code location for symbols. Usually, nrepl's eval
will return nil for symbols defined in code loaded by other mechanisms
that via the system classloader. For nrepl.el, this breaks functions
such as nrepl-jump. Including this slot allows other middleware to
specify which classloader should be used for loading resources.