Generate Heading IDs

This PR is a modification of #117 with the following differences:

1. Don't generate extraneous markup (no `anchor` tags, just `id` attributes)
2. `name` attribute is obsolete in HTML5. `id` attribute is recommended
3. `id` attribute accepts any non-white-space character per HTML5 spec (and is backwards compatible)

Changes:
* revert the anchor options (`anchors`, `anchorClass`, and `anchorContent`)
* revert the anchor element itself
* simplify the heading-text-to-id logic