Add "defined in file" construct.

[realyze]

Hi,

I came across a situation where I have non-trivial setup (basically a short description of a db table) in my feature file background and I would like to be able to share this setup between feature files (to keep things DRY). I know I could put the setup in the steps implementation, but I'd prefer to keep it in the feature file, because it's easier to see the how the output of certain features correlates with what's in the setup.

I couldn't find any way to share background between features, so I propose we add a new construct: "defined in file" which could be used as a part of a step description and would allow us to specify where a common setup is located (the step implementation would then get a parsed table).