Yarn workspaces support

Hello there, thanks for this project.

I use Yarn and Lerna with Yarn workspaces. But it disallows install packages to root space without `--ignore-workspace-root-check` or `-W` [flag](https://yarnpkg.com/en/docs/cli/add#toc-yarn-add-ignore-workspace-root-check-w) and I got an error:

```
$ commitizen init cz-lerna-changelog --yarn --dev --exact
Attempting to initialize using the npm package cz-lerna-changelog
yarn add v1.13.0
error Running this command will add the dependency to the workspace root rather than the workspace itself, which might not be what you want - if you really meant it, make it explicit by running this command again with the -W flag (or --ignore-workspace-root-check).
info Visit https://yarnpkg.com/en/docs/cli/add for documentation about this command.
```

Unfortunately, I see no any other way, except to add another flag to [getYarnAddStringMappings](https://github.com/commitizen/cz-cli/blob/master/src/commitizen/adapter.js#L132). Perhaps there is another solution, but any solution to this problem would be great.

P.S.: I installed and configured packages manually, and everything seems to be working fine.

Start in src/commitizen/adapter.js at getYarnAddStringMappings, as linked in the issue, and trace how commitizen init builds the Yarn add command. Verify the resulting command supports Yarn workspaces by allowing the dependency to be added at the workspace root, then run the relevant existing checks or reproduce the reported command to confirm initialization succeeds.

Scritto dal modello di indicizzazione a partire dal testo della issue.