Allow log.io= to properly update underlying stream.

Stream logger output to StringIO buffer instead of STDOUT for tests
Add yard markups command to list available markup types
Add `yard display' command.

Displays an object in the registry with any formatting. Examples:

Display YARD::CodeObjects module as text:

    $ yard display YARD::CodeObjects
    ...output here...

Display same object as HTML, pipe into .html file:

    $ yard display -f html YARD::CodeObjects > codeobjects.html

Custom templating with -t and -p also work (as in yard doc).