Support subdirectories when generating features

I always organise my features into directories but currently spinach doesn't support this, when they're generated they're always placed in the root features folder. This patch will let you use a slash in the feature name to specify a directory e.g.

Feature: admins/Role management

    $ spinach --generate
    $ ls features/steps/admin/
    role_management.rb

I haven't added a test yet, thought I'd see if you're open to the idea first. I did consider taking the folder from the file path of the feature file but this is a bit awkward because the parser that generates the feature doesn't know anything about the file (just the content).