Feature - #185 - stagehand integration

Stagehand integration works like a somewhat primitive wizard:

1) select 'Dart: Stagehand' command
2) select template name
3) select target directory

If stagehand is not installed, it will be installed and the
template generation will be aborted. The user will have to
restart the process in order to generate a template.

**TODO**

- [x] test on Win
- [x] test on OSX
- [x] test on Ubuntu