Text, link, and node formatting and interaction

-Section titles are by default invisible.
-A maximum of five links are highlighted from the user's history.
-Section titles are partially transparent if they are in the highlighted
path from the user's history, connected to the current section the user
is viewing, connected to a node the user is hovering their mouse over,
or the source node of a link the user is hovering their mouse over.
-TODO: Nodes are centered in the middle of the SVG, not the middle of
the document.
-TODO: Arrows don't resize properly.
-TODO: Better link and node colors need to be established.