Add result type :return to custom eval

A new :result-type for custom evallers that allows to pass the evaluation's result to an arbitrary object instead of displaying it. (For those familiar with Clojure development on Emacs, this would be analogous to CIDER's eval-and-get-value, with the additional benefit of being async.)

Such a facility would be rather useful for plugin development. It grants the caller easy programmatic access to the result of a custom eval, allowing them to behave accordingly.
For example, a factoring command could listen for the result of an analysis, then use its value to produce a snippet and move the cursor to the most convenient position in the editor.

The current implementation enforces a one-to-one correspondence between handlers and behaviors, because a custom evaller of type :return always raises the same trigger. An immediate improvement that I think rather desirable would be to allow the caller to also specify which trigger to raise upon successful eval, so that a single object may manage multiple events. (Edit: this improvement is now in to the branch.)

[Here](https://www.refheap.com/43096) is a simple test case for this PR. If you are interested, I will extend the patch with the addition mentioned above and a few other things (such as returning the location, which is currently unused) before merging.