Include external source files in code sections

Using { include="path/to/file" } in the attribute block for
code sections it's possible to pull in external code rather
than do it all internally.
add symlink for preview
smaller neater css for code examples
New post using includes for first time

And preview script that might get scrapped.
Rename drafts with fake dates
Allow rendering drafts to alternative location

Two new commands "drafts" and "alldrafts" are the equivalent of "build"
and "rebuild" using posts written in drafts/ instead of posts/. They
require dates in the file name or the published: field so just use fake
ones --- the quickest way.
update scripts for fake dates on drafts