Add ability to open default settings for viewing

The default settings file (Packages/Dart/Support/Dart.sublime-settings
isn't meant to be edited by users, so open it as read-only. In fact,
it's opened as a scratch view with no actual path, just a buffer
name.
Update README
Merge pull request #152 from my-personal-forks/readme

Update README
Merge pull request #151 from my-personal-forks/settings-2

Add ability to open default settings for viewing