Add --yarn option to use yarn instead of npm

[glensc]

Fixes #289

use yarnInstall if --yarn option is used:

• https://yeoman.io/authoring/dependencies.html

[glensc]

I don't know how to test this without code being published in npm registry.

[oklas]

It is possible to install package for testing. Package may be installed from local folder or from tarball, both yarn and npm.

[glensc]

Probably more changes needed, as this did not have any effect at all

[oklas]

Perhaps yes

[oklas]

Moreover, I do not see similar testing for npm either. But I noticed that the readme template generator should also be changed here

[glensc]

PR for readme sub-generator:

• Add yarn/npm install to readme on yarn condition #294

let's get that resolved and merged first.