Add an executeFromWorkspaceRoot flag to allow yarn commands to execute from the root of each workspace

[ajhool]

After I build each workspace, I zip each result into the workspace's build/dist folder (my yarn script is called package). For package to work properly, it needs to be executed from the workspace's root dir. Usually, I call cd path/to/workspace; yarn package. Currently, executing wsrun package will not implicitly perform a cd path/to/workspace, so the relative paths used by package/zip are incorrect.

If there is interest in this feature, I can put together a PR