Optionally install gpt.js by default

[cobbdb]

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.