0.7.4

• Modify interactive processor to use slug rather than name to identify interactives.
• Modify video processor template for YouTube videos.
• Modify boxed-text processor to have optional type parameter.
• Update style error message to include line numbers.
• Dependency updates:
  • Update python-slugify to 1.2.5.
  • Update setuptools to 39.1.0.
  • Update sphinx to 1.6.6.
  • Update sphinx_rtx_theme to 0.3.0.