add `anchor` helper to add section anchors
convert <h2 class="content-subhead"> to use {{anchor ".."}} helper
add anchor image to content with anchor urls
Rename `anchor` helper to `sectionHeading` to be more clear
Position anchor links at the right center, inside the headings
Merge pull request #13 from ericf/section-anchors

Section anchors updates
Merge pull request #153 from tilomitra/section-anchors

Add section helpers to all <h2 class="content-subhead"> elements

Fixes #152