2.0 - Support for handling invalid parameters (#23)

[edno]

Update behaviour for handling invalid parameter case.

3 behaviours supported using module configuration:

1. default: if parameter/variable does not exist then it is not replaced (see discussion inconsistent variable or key not found  #23), ie {{not_valid}} will be {{not_valid}} at runtime
2. nullable: when onErrorNullable is true then invalid parameter/variable are replaced by null, ie {{not_valid}} will be null at runtime
3. exception: when onErrorThrowException is true then invalid parameter/variable throws a GherkinParamException at runtime

⚠️ This change requires to transform GherkinParam into a module. This is a breaking change for existing setup).

modules:
    enabled:
        - Codeception\Extension\GherkinParam
            onErrorNullable: false|true
            onErrorThrowException: false|true

Merging this PR will trigger version 2.0

[coveralls]

Coverage decreased (-7.3%) to 92.683% when pulling 01eb72c on error-cases into 0720975 on master.

[coveralls]

Coverage decreased (-7.3%) to 92.683% when pulling 01eb72c on error-cases into 0720975 on master.

[coveralls]

Coverage decreased (-11.5%) to 88.506% when pulling 6df3b55 on error-cases into 0720975 on master.

[gimler]

Works as expected good work