Meta: npm installer improvements

[rtfeldman]

This is a meta issue to collect potential improvements to the npm installer. If you think something is missing, please open a separate issue!

• [ ] #208 Check for downloaded binaries and don't bother re-downloading upon reinstallation
• [ ] #210 Make package version string parsing more flexible, to work around an npm issue
• [ ] #224 Host binaries through GitHub releases instead of Bintray
• [ ] #226 Improve npm's error message in case of permissions issue
• [ ] #205 The installer should work with --ignore-scripts=true

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.