make library usage more convenient
make library usage more convenient

HTMLi:
- add a high level #parse method
- #format:
  - if :collapse is set and tree is not height-tagged
    (respond_to? :height), pass it throuh #tag_height
  - have :collapse to default to nil (departing from
    the command-line default --collapse=2 which is kept)
  - default :layout to MapLayout (in #mktree actually).

So now HTMLi.parse(<io or string>) and
HTMLi.format(<tree of some HTMLi layout>) can be conveniently
called without much boilerplate or long list of magical options.

csabahenk/HTMLi