Add ability to run a single test/feature

[rcernich]

When doing test development, some suites may take hours to run, which makes things difficult if one is trying to debug a single test.

[vrutkovs]

That would depend on #58 - pass parameters to behave to filter out features/scenarios by tag or start a scenario at particular line and a file

[rcernich]

I understand we can use @wip to run single features/scenarios, and I'm using this as a workaround, but this requires modifying the actual tests. Other test frameworks allow you to include or exclude specific test cases from a test run.

[vrutkovs]

We're limited to behave as a testing framework, so the options are described at http://pythonhosted.org/behave/tutorial.html#controlling-things-with-tags:

1. Run scenarios marked with specific tag(s): -t @tag
2. Run scenarios with a specific tag(s): -t ~@tag
3. Run a particular scenario: features/name.feature:<line of the scenario>

Anything else we might want to add?