Add npm analytics

[jasonkarns]

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

• [x] scrape data #17
• [x] render analytics pages #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