Allow overriding cucumber_steps_glob definition for local requirements

[JESii]

Allow overriding the default glob definition for b:cucumber_steps_glob by exporting the CUKEFILES environment variable. As noted in the updated README file, this can be necessary when working in a large, multi-project environment when each project has it's own features/step_definitions/ where steps specific to that project are stored.

My working environment has half a dozen such projects and I was receiving many "Multiple matching steps..." error messages because of similar SDs for the different projects. Our cucumber yml files took care of use through different profiles for each project, the vim-cucumber bundled them all together with the default glob definition.

(NOTE: I've squashed all the commits in my local project and pushed to this branch, but there are still multiple commits included here... not sure why, but I'm happy to fix if that's needed.)