Move unused layouts to `_ignore` directory
Move header stuff to `_includes/header.html`

I just hate editing the `layout` unless it is necessary; this allows me
to tweak only the header instead of the entire layout.
Add support for global css/js
Use markdown instead of HTML links
Rename and reorganize Slate layout files
Split up `slate-custom.css`
Ignore old and unused stylesheets
Move included "list items" to `lists` subfolder