Default exports don't work

[swansontec]

If an entry point has a default export, like export default 1, this plugin's export * from 'entry' strategy won't include that.

Apparently * doesn't include the export named default. You can use the rollup REPL to play around with this.

Possible solutions:

1. Include an export { default } from './b.js' line in the generated output, iff the input has a default export.
2. Finally get around to implementing Consider copying the file content instead of exporting it #7, in which case we will work correctly regardless.

[swansontec]

Detecting whether or not an input has a default export is pretty insane, since it requires a full parse of the source text.

Instead, let's add a config option, exportDefault, which can be set to either a boolean or a file name. If this option is present, we'll go ahead and add an export { default } from '...' line with either the input file name, or whatever name the user supplies.