Add npm analytics

[jasonkarns]

As nodenv formulae are installable via npm as well, we should include npm download/install stats alongside homebrew

• scrape data Generate analytics data from npm downloads #17
• render analytics pages Render npm analytics #19
• render analytics json/api
• add npm references to formulae pages

[jasonkarns]

PRs to open upstream:

• table widths need to be min-width 100%
• "depends on" block should be hidden when empty