Add deprecation warning for old paths.markdown option
Extract new Markdown class to encapsulate markdown splitting and marked
consume the new Markdown facade
add deprecation warning for use of  accessor on Page
pass options to marked when compiling, rather than pseudo-static setOptions
Merge branch 'extract-facade'

* extract-facade:
  pass options to marked when compiling, rather than pseudo-static setOptions
  add deprecation warning for use of  accessor on Page
  consume the new Markdown facade
  Extract new Markdown class to encapsulate markdown splitting and marked
edge cases for #get
use _.result
consolidate where config property checking occurs
verify archive config earlier
verify index config earlier
Create Config object to cleanse incoming options
Use Config object for validating Feed options
Feed no longer needs to do any option-validating or logging
tweak what Config returns for invalid keys
Config#forArchive
Factory validates config and builds Feeds
Use new Factory and updated Config to create Feed
Factory validates/creates Archives
Config#forArchive