Rethink playpen configuration

[budziq]

As discussed in https://github.com/azerupi/mdBook/pull/338#discussion_r123618836

Expanding the playpen config to make features opt in or opt out might be a good idea.

Discussion points:

• Should the features be opt in or opt out like (clipboard support, editable, resetable if editable, code runnable)?
• Should playpen styling be a plugin too? Explain alarmed ferris convention in chapter 1 for code that doesn't compile on purpose book#55
• Playpens are currently tightly coupled to html/js. Code snippets would be relevant in other rendering contexts as well but not all features would be applicable
• Alternative editors (mathjax?)
• Consider alternative playpen backends for remote code execution
• currently rust has better support for playpens than other languages:
  • remote execution
  • code hiding with '#'