Refactoring In Progress

- Don't remove single paragraphs inside <blockquote>,
  <li>, <dd>, <th>, <td> by default; They do have
  slightly different semantic meanings.

- Add explicit type coercion and guards around the
  code base.

- Combine ParserBase into Lexer.js for better code
  reuse; Serializing AST into BBM will need knowledge
  of lexical tokens.

- Remove "disallowed" from the default options; There's
  better way to manage permissions than this.

- Shorten utils.js to __.js. Uses two underscores to
  prevent crashing from REPL use case.