Prevent trailing slash in config output
Add conditonal paths for sass on Windows
Support Windows path but don't break URLs
Specify file open mode
Merge branch 'windows'
Fix bug in plugin call if using bound methods

Bound methods used as plugins would not have the expected number of
arguments (there's self too) although it does not change the way we
should determine the calling convention!
Add option to unregister externals
Support static not in the site's static folder

This is useful to give the user the option (in plugins) to shadow
existing static files with other static files.
Optimize static discarding (don't optimize)
Remove memoize for static and replace w/ accessor

An accessor means that plugins can modify the list of static files after
we have initialized those.

This lets plugins alter the static files present, possibily replacing
them with other files, removing, or adding other files.
Add a test for static de-registration
Add documentation for plugins