Fix #316 - Add 'Dart: Show Output Panel' command palette item.

Scripts and server apps run from within ST will most likely print
text to an ST panel. This panel may be hidden accidentally by the
user (for example, by pressing Esc). This new command palette
item will reopen the main output panel used by the plugin.
Merge pull request #318 from my-personal-forks/feature-show-output-panel

Fix #316 - Add 'Dart: Show Output Panel' command palette item.