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.