Add sequence args

[Ratsch0k]

Hi @CoreyD97,

this PR would add the ability to pass arguments into sequences.
With the syntax X-Stepper-Execute-Before: sequence: var1=value1; var2=value2 or a dedicated argument header X-Stepper-Argument: var=value this allows passing values into the sequence. The argument overrides the global variable of the sequence.

With this addition, some more complex and different request sequences can be tested with Stepper. For example, I sometimes had the following behavior: One request creates some object based on the request's data. But the response does not include the created object. Another request only returns the previously created object.

Testing this behaviour is usually very annoying but with these changes this can be easily tested with Stepper.

1. Create sequence with the first request. Use global variables in request
2. Test the second request via Repeater, Intruder, Active Scanner, etc. and pass the payloads to the first request via the argument

I also added a short explaination of these changes to the About Tab.

Could you kindly review these changes and merge them if everything checks out.
This feature could be very nice.