Preload visible links in viewport

[daun]

Description

• Install an IntersectionObserver on the viewport
• Preload all links inside the viewport
• Makes more sense than quicklink: better integrated, less polyfills, less options
• Not enabled by default: added an option preloadVisibleLinks to enable it
• Tested and working like a charm
• Adds ~0.7K to the bundle (vs. 2K for quicklink)

Drive-by improvements

• Check for bad network conditions (data-saver or 2G) where supported
• Add an option preloadHoveredLinks to allow disabling that feature

Checks

• The PR is submitted to the master branch
• The code was linted before pushing (npm run lint)
• All tests are passing (npm run test)
• New or updated tests are included
• The documentation was updated as required

Additional information

Closes #84

The base branch was changed.

[daun]

I've rebased on the current master and updated the readme to mention the focus feature as well.

@hirasso Feel free to either reapprove or wait until you had a chance to give it a spin :)