Support customizing layout contents

[sloshy]

There are some useful Jekyll plugins that sometimes require modifying the contents of your layout. For example, https://github.com/toshimaru/jekyll-toc requires modifying the {{ content }} section to include the table of contents.

Should look into making tweaks such as this, or other obvious tweaks while we're in there, possible per supported layout.

[sloshy]

Originally requested by @jackkoenig