Fix menu links for "welcome" anchors.

In the `index.html`, `{{ welcome_content |markdownify }}` generates name
attributes for header tags like "my-title", whereas `{{ post.content }}`
generates name attributes like "my_title".

This fixes the links for only the welcome menu items so that the anchors
in the menu match the different name format.
Merge pull request #423 from jwpage/welcome_anchors

Fix menu links for "welcome" anchors.