add support for ivy and completing-read

[peterhoeg]

This is what I'm using on my side as I don't use helm.

I can definitely do up some documentation, but before I start writing a novel, I want to be sure you're OK with this.

A few changes here:

1. split out helm support to a separate file
2. add support for ivy via separate file
3. add support for completing-read (for use by vertico)
4. use Cask to run tests
5. fixes #10
6. add support for running tests via make
7. rename test file to be in line with the buttercup standard structure

I have a change ready to PR for melpa for the helm and ivy packages.