plugin init or reset hook

[akvadrako]

Some rollup plugins keep state that needs to be reset after each run. For example, CSS extractors will collect styles on each call to transform() and dump a merged CSS file beside bundle.js on the final call to onwrite(). I'm writing one now for stylis.

When running rollup watch, I cannot find a reliable and documented way to detect when a new run starts. It would be nice if a plugin.init/reset() hook was added.

[guybedford]

A buildStart and buildEnd hook have now been added in #2208.