ember-cli-google-analytics: Expand and document gtag.configure() options

[fdeters]

Hi there, I just have a small addition to the docs detailing the options you can pass to gtag.configure(). I based them off of my understanding from reading the gtag service code, so please feel free to edit them if needed.

I think this is a good contribution because I personally needed to dig through the source code to find forceEnable to check that gtag.configure() was working correctly. Putting it in the README makes that a little easier.

[fdeters]

Update: The second commit takes this a little beyond the original scope of this PR, so I'm happy to separate it if needed.

I added a debugMode option to gtag.configure, which can be used to turn on the gtag API debug mode.

This change just adds it as a new option that can be passed into either .configure() calls or the config/environment configuration, just like measurementId and forceEnable.

[hilljh82]

Thank you for this contribution.