Upgrade BBM.fn.toHTML() prototype.

- Added pretty-printing (indentation) support.
- HTML-specific printing options implemented. They are:

  ; rmNL (Boolean : false)
  : Remove line break characters from text node output.

  ; maxAttrChars (Integer : 2048)
  : Limits the maximum number of characters allowed in
    HTML attributes and identifiers.

  ; headerOffset (Integer : 0)
  : Increments header levels by one, up to a maximum of h6.

  ; XHTML (Boolean : false)
  : If true, generates valid XHTML output in place of HTML.