Add generator.installDependencies to install bower and npm dependencies

With the new concurrent installation of bower/npm dependencies and the option to skip the automatic installation, the code became a bit too large to be copied around comfortably.

generator.installDependencies accepts three options:

```
// Options:
//  - npm: bool, whether to run `npm install`, default: true
//  - bower: bool, whether to run `bower install`, defaulkt: true
//  - skipInstall: bool, whether to skip automatic installation and just print a
//                 message to the user how to do it, default: false
```