William/handle default

[swansontec]

This is a WIP solution to #12.

While this solution probably works in like 99% of cases, it uses a gross regular expression to determine whether or not a file has a default export. This will obviously fail if the file has the words export default inside a string, comment, or other non-code thing.

Instead, we should use a library like @babel/parse to properly analyze the source text to determine whether or not it has a default export.

[swansontec]

Rollup itself now produces a list of entry point names, so we can use that to finally implement this feature.