Ability to get unbundled cjs/es modules as in version 0.4.4

[gamtiq]

This is re-open of #514.
Unfortunately the issue takes place in version 0.13.
See gamtiq/microbundle-issue-514 to reproduce.
Related pull request: #592

[developit]

Ideally, I'd like for Microbundle to use Rollup's built-in multi-entry support to achieve this.