Fix example app issues when installing local copy of the arc

[MarquessV]

When installing a package from a file, yarn seems to copy the entire folder, including any dev dependencies that might have been installed. This meant that if you built the example app after installing the dev dependencies for the arc, duplicate versions of react would run in the example app and cause it to crash.

To fix this issue, a new postinstall script will run that will prune any dev dependencies out of the example apps copy of the arc.

[jkhusanov]

checked the branch - works well