Update micromatch dependency

[lukastaegert]

This is a first attempt at updating to the new micromatch 4. My fear is that this would be a breaking update as I think I have spotted some object rest/spread operators in its dependencies, but we will see what CI says on Node 6. Otherwise I wonder if we should consider moving the "createFilter" functionality to core Rollup (and make it throw if we are on a browser build) as there, we could use TypeScript to transpile the spread operators to something more downward compatible.

[lukastaegert]

And here we go, this would remove Node 6 compatibility.

One advantage of moving it to the plugin context (i.e. this.createFilter) would be that Rollup already bundles this part of the pluginutils anyway, so reusing it by plugins would be a natural choice. Feedback welcome.

[lukastaegert]

Ok, so after some deliberation I decided the way forward is to bundle a Node 6 compatible version of micromatch with rollup-pluginutils (by means of transpiling it via the TypeScript compiler to ES2015). That way, we can guarantee compatibility across the ecosystem but profit from the better performance and reduced size. Especially Rollup itself will profit as it bundles this specific part of the pluginutils for some watch-mode features.