Hacktoberfest 2019: four PRs to Parcel

So here it goes — four PRs submitted for this year's challenge!

It took me several days to find issues of the right size, debug them down, and submit the solutions, but it was worth it. :)

Honestly, every PR here is quite small, but I'm still satisfied with the impact.

1. Add script href as a viable attribute for dependency detection
2. CONTRIBUTING.md: add a snippet re: how to run the dev version
3. Purge dist dirs before bundling
4. Fix: set pragmafrag for jsx in v2