Dartdoc preview fixes

Various presentation fixes.  Notably:

* newline fixes: http://youtrack.jetbrains.com/issue/WEB-12049
* basic markdown support (integrates https://github.com/JetBrains/intellij-plugins/pull/12)
* improved signature preview (to be consistent with DartEditor)
* improved external doc handling (still need to fix http://youtrack.jetbrains.com/issue/WEB-9803)

NOTE: this is just a step.  Lots of potential refinement!