Added section name highlights

[thomaswebbers]

This script will highlight the current section names in the navigation

[NielsGrootObbink]

The following issues should be fixed before the pull request is accepted:

• Next section should be highlighted halfway through the page, not after the section hits the page top.
• console.logs should be removed
• Script should be included in the index.html file
• Active state should be the same as hover state (circle should be filled)
• Update README.md file with instructions and script tag.