Change header levels while transforming markdown

[jeroenheijmans]

It would be nice if each blog post .md file would be self-contained, and starting with a h1 heading. That is the "right" way to set up that content. Plus it will help with some day creating a page-per-blogpost when the /devlog page grows too large.

But, when we do this it would be necessary to be able to bump down each heading level when transforming markdown, otherwise /devlog will get multiple h1 headings (one per blogpost), and I don't like that: it should have at most one h1. So, the main to-do's would be:

• Change all existing blog posts to start with h1 headings and consistently move down levels
• Change markdown transformation so /devlog won't get multiple h1 headings as a consequence

Additionally, let's take along some other relevant changes:

• Change the "author + date" line in each blog post to not be a heading anymore, just a highlighted, emphasized tag line below each h1
• Change the level of all other headings to be appropriate