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!

• The post install hook for npm package fetches the elm binaries every time yarn/npm install is run #208 Check for downloaded binaries and don't bother re-downloading upon reinstallation
• package-lock.json may cause 404 errors in install.js #210 Make package version string parsing more flexible, to work around an npm issue
• Switch from Bintray to GitHub releases for hosting npm binaries #224 Host binaries through GitHub releases instead of Bintray
• Help users fix npm permissions issues #226 Improve npm's error message in case of permissions issue
• npm installer should work with --ignore-scripts #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.