Prevent cucumber yml being read twice

This fixes #217.

We were rereading cucumber.yml for every profile we were merging in because we were creating a new `profile_loader` each time we were parsing options.

I've used a global variable and erb to test how many times the cucumber.yml is parsed: https://github.com/cucumber/cucumber/compare/prevent_cucumber_yml_being_read_twice#L1R231

Does anyone have a better idea?