Labels on edges

[mileswest07]

Request to allow developer to enter text, an image, etc. along edges within the tree, as desired.
This could be used to show prerequisites, costs, branching research path names, etc. That data could be saved either as another JSON object with edge nodes, or else children within the JSON source file (demo_tree.json) could include an edge property to use to link to its parents, with an HTML string that could include image icons, etc.
Lines ~63 - 69 of techtree.js would need to be rewritten to allow this.