Add helpers
Fix #161 - Add ability to run pub commands from pubspec.yaml

If the active file is pubspec.yaml, the user can:

- press F7 to run `pub install`
- open the command palette and find 'Build: Dart: *' pub
  commands

Also, saving a pubspec.yaml file will run `pub install`.
Change command name
[Pub] update --> upgrade
Merge pull request #168 from my-personal-forks/pubspec

Better integration with pubspec