Convert markdown image syntax with [bg] into directives

[yhatt]

This PR will add markdown-it plugin to convert markdown image syntax with [bg] into directives (#4).

ToDo

• Basic feature of Marpit background image plugin
  • Hide image from rendered HTML when it includes bg in description
  • Assign backgroundImage spot directive
    　
• Deckset style sizing
  • Assign backgroundSize spot directive with percentage
  • Assign backgroundSize: contain spot directive with fit keyword

EDIT: We will split the implementation of sizing syntax into another PR.

We will implement the filter and multiple background at another PR, by using a SVG data URI scheme to background image.