Config doesn't need grunt
Config has defaults
specify shallow merge
slim defaults for test
options from grunt to MarkdownTask through to Config
instatiate Config outside MarkdownTask
Add Config#forSiteWrapper
use Config#forSiteWrapper
add Config#destDir
use Config#destDir
Merge branch 'config-options'

* config-options:
  use Config#destDir
  add Config#destDir
  use Config#forSiteWrapper
  Add Config#forSiteWrapper
  instatiate Config outside MarkdownTask
  options from grunt to MarkdownTask through to Config
  slim defaults for test
  specify shallow merge
  Config has defaults
  Config doesn't need grunt
Factory::siteWrapperFrom
use Factory::siteWrapperFrom
NullLayout no longer needed