Optionally install gpt.js by default

Append `gpt.js` if not already included. Nice quality of life feature to simplify setup.

For example, where calling `show.all()` would include quick and fast check if gpt has already been installed and setup. If not, then wrap the action in `googletag.cmd` and append the `gpt.js` script. Will have to remember if we've already dropped in the script to avoid duplicates.
