More explicit chaining of queries

[vnaydionov]

I'd like to propose an externsion to the testing feature.
Since the spec .yml file essentially maps to a linear chain of calls, it would be useful sometimes to alter the execution order of those queries.

1. For example, if some call fails an assertion there might not make much sense to run the rest of the call chain. This could be signalled somehow in the tests section.

2. Another case when further execution may better be stopped is when there's been a problem with computing a variable value from a response.

3. Yet another thing that might appear useful would be possibility to retry certain call until a user-defined condition is met, maybe specify it in a similar way that assertions are specified. Of course here we would need to be able to set time limits and inter-call delays.

Together these features would allow for more complex scenarios with objects having their state changed by some background tasks.

[supakeen]

Steering close to a DSL 👍 if it is decided that things will move that way then #51 is probably part of the same change.

[camilamaia]

@vnaydionov I've created a separated issue for the 3rd point: #298

[taconi]

Perhaps can be added a parameter --fail-limit. I can take this task.