Possibility to filter stories with CLI param

[jack-sf]

Feature request: For example, npm run happo --only Button would only compile and run stories that have Button in their story name (or in the story filename).

[trotzig]

This is in fact a bug/quirk with the happo-plugin-storybook plugin. If you use the default Happo way of defining examples (via -happo.js files), you can run with the --only flag.

[jack-sf]

Yeah, but we could somehow enhance this plugin so it supports this --only option as well.