Accessibility improvements for npmjs.com

Many accessibility improvements have been deployed to npmjs.com. Highlights include:

• Site-wide improvements to color contrast, text resize, and support for users with low vision.
• Improvements that enable keyboard-only access including visual tracking of the focus indicator.
• Improved support for assistive technologies including screen readers.

Your feedback is welcome! Please share feedback on the accessibility community discussions page and learn more about GitHub accessibility at accessibility.github.com.