Add support for customizing which index paths are invalidated

[fenichelar]

Purpose: Optimize invalidation costs by only invalidating the specific paths needed.

Configuration options:

indexFullPath: false            // Invalidate index.html paths (`/foo/index.html`) (default: true)
indexTrailingSlashPath: true    // Invalidate index.html trailing slash paths (`/foo/`) (default: false)
indexNoTrailingSlashPath: true  // Invalidate index.html no trailing slash paths (`/foo`) (default: false)

Backwards compatible: Same default behavior. indexRootPath removed from readme but still functional.