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
Add .path_to_docgen to SDK class

- refactor
- add sdk.SDK.path_to_docgen property
Add ':' after Dart in build system names
Add basic docgen tasks to build system

- Build: Dart: Generate Documentation (from project dir)
- Build: Dart: Serve Documentation (from project dir)
Edit manifest
Merge pull request #168 from my-personal-forks/pubspec

Better integration with pubspec
Merge pull request #173 from my-personal-forks/manifest

Edit manifest
Merge pull request #174 from my-personal-forks/docgen

Basic support for docgen