add support for plugins
add err callback for plugins
add optional glob expansion plus auto-requiring for plugins
Support single category and tag
Merge pull request #325 from adjohnson916/master

add support for plugins
externalize plugin support, integrate resolve-dep, and add tests for
loading plugins. see issue #326 and pull #325.
Merge pull request #327 from adjohnson916/master

externalize plugin support, integrate resolve-dep, and add tests for loading plugins
Predefine permalink

  'date': ':year/:month/:day/:title',
  'none': ':category/:title',
  'pretty': ':category/:year/:month/:day/:title'

Default permalink from assemble.options.permalink
Merge remote-tracking branch 'upstream/master' into permalink