No warning given on /tests folder overwrite

[jperezov]

Currently, npm install intern-ui overwrites the /tests folder.

While the README has a large, very clear warning regarding this, users are still
likely to run the install script without actually reading the README. Additionally,
upgrading this package will also overwrite the folder.

A flag should be added to the install to give users a choice as to whether they want the
postinstall script to run (this is the script that overwrites the folder).