StuffPages 0.7.0

• new template system (string.Template) using $variable notation instead of {{variable}}
• output_dir is now relative to _stuffpages directory
• internal links fixes
• html output fixes
• default theme CSS improvements and fixes
• 4 new themes: selenized-light, selenized-dark, selenized-white, selenized-black
• switched to different code highlighting plugin (pymdownx.highlight)
• updated config default settings