Design a plugin(-like) architecture for test drivers

```cucumber
Feature: test drivers as plugins
In order to support multiple test frameworks
As a developer
I want to be able to add new test drivers easily, without needing to extend the core
```

### Summarized

We're seeing quite some pull/feature requests for adding support for test framework X, Y and Z (e.g. #19, #20, #129, #142). In order to keep the core of cucumber-cpp maintainable I believe we need to be able to keep the support code for additional test drivers isolated from the rest. With isolated I mean: not intermingled with the rest of the source code, e.g. preferably grouped in a single directory per driver. Additionally this may help to make it possible to support out-of-tree test drivers.