[WIP] Refactored ReplaceStrategies into Commands

Fixes #19, almost completed

Tests are currently failing because I didn't yet updated the 4 `EditorSpec` manipulation methods. I'm not sure how to properly write test with phpspec about correct command being created inside the related Editor's method. Waiting for some feedback on this point :wink: 

Moreover, I'm not really satisfied with the current solution because a `LineContentConverter` is instantiated each time a Command is created.

I hope the current architecture isn't too awful and is now opened for some review :smile: 

- [X] Create commands
- [ ] Fix tests
- [ ] Remove replace engine and strategies




It executes the wanted command:

1. add a mock command
2. try to retrieve it
3. it should not throw an exception

With phpspec, the argument passed to the test methods are double tests: the framework creates an object which corresponds to the given type. So you can pass directly the `Command` interface