Proposal: Pluggable Matchers

For our purposes, it would be valuable to have cells in our decision table which can do set logic.

Rather than patching the Table class directly, I propose to first make pluggable matchers, allowing adding new matchers to Table without subclassing it.

[details here](https://github.com/lastobelus/rufus-decision/blob/pluggable_matchers/PLUGGABLE_MATCHERS.md)