Move to separate index generator

No changes to appearence yet. This is a blank slate for customization.
Aesthetic reformat of the index page.

Split files by type, catalog-inspired directory nav, hide dotfile, build, cache.
A basis for adding debugger and editor support.
Remove unix-compat alt way to get modified times
Print Elm version on command line
Don't separate out HTML5 files, per @evancz
Get rid of unused imports
Fix error in which files within folders errored
Mess with style, fix directory links