Add support for happo dev command

[jack-sf]

Just like in happo run, happo dev could use the runner from the plugin, which would inform the happo.io script:

• when initial compile is done
• when compile on reload is done

Then, together with filtering stories that should be done, we could have a nice developer experience with Happo, when a dev can work only on one component, and have a live screenshots of it in all the happo targets.

[monokrome]

Same here. Seems like it just freezes after first run :o