Feature.max_length should consider only assigned scenarios

[Janghwan Kim]

Hi.

I have a scenario with a very long step sentence -- not a good design,
but had to test long input -- and I found the terminal output is very
ugly, even if I don't select the scenario.

When running lettuce with selected scenarios with -s option,
I would prefer Feature.max_length to consider only the selected
scenarios.

Reading the code (v0.1.35),
I found the scenarios selected with -s option is only valid upto the
Runner, and Feature instance holds the entire scenario set.
Perhaps you can add scenario argument to Feature.from_file method to
limit the scenarios, since the unselected ones are not intended for
the test?

Thanks.