current-directory not needed, use uiop instead

   Add setf expansion for getcwd
Make tags indices URL absolute
More robust directory changing

     - Doesn't fail when the directory is missing a trailing '/'
     - When directory doesn't exist it signals an informative
       condition.
Merge pull request #56 from PuercoPop/misc-fixes

Misc fixes
Fix validation errors.
Merge pull request #59 from Ferada/validation-fixes

Fix validation errors.
fix link to chiptheglasses
Merge pull request #60 from nathanielksmith/fix-link

fix link to chiptheglasses
Fix buglet in readable theme's post template.

Thanks to Matthias Wimmer for the report and fix.
Bugfix: Feed should be an abstract class.
Remove scaffolded parallel plugin.
Tentatively factor deploy method into git-hook plugin. TODO follows...

* Deploy :after plugins probably need revision now, and coleslaw-heroku.
* README, HACKING need updates. Plugin-api.md too.
* NEWS needs a carefully worded entry.

Is that what we want?
Incremental compilation is done, note some new goals.
Add some error-handling notes.
More notes.
Some TODO notes.
Bugfixes.
Half-way commit to getting hardcoded paths out of templates.
Tracking TODOs in the Hacking file now.
More TODO notes.