Rework config loading

[marvinhagemeister]

Copying from slack:

• load configs up-front before calling into build() or start()
• start() sets up file watcher for package.json. Successfully parsed file changes emit a configChange event on options.bus
• middleware() listens for that event, and invokes a special configUpdated(newConfig) hook on all plugins.

(If necessary, we can also listen for the event and fire the hook in the second RollupPluginContainer used by the CSS middleware)

this also nicely keeps the initial package.json + wmr.config.js parsing as a one-time (non-watch) thing that happens prior to build()/start().

[developit]

I started on this a little bit in #169 (needs to be factored into a separate PR). Still needs the package.json loading.

[marvinhagemeister]

Closing since the config file handling has been greatly improved since this issue was created. We now support wmr.config.* config file 👍