Encapsulated multiple tracker handling

[justinsa]

• The previous design for supporting single or multiple trackers forced copy-paste coding and special cased the scenario of a single tracker versus multiple trackers. This change makes account handling generic and encapsulates the logic for how multiple trackers are called to a single location with backwards compatibility preserved. GA commands that support multiple trackers will be trivial to add support for now, but will require additional tests in order to onboard properly.
• Standardized the generation of the create script tag for Analytics.js to make generation consistent without special casing on whether there were one or many trackers defined.
• The setAccount configuration method now supports a more complete set of formats. Those are: 1) no argument / undefined / false will unset the accounts previously defined, 2) an array of tracker objects, 3) a single tracker object, and 4) a single tracker identifier. This preserves backwards compatibility.

[justinsa]

@Karpo72 - this PR will make it easy for anyone to add support for multiple trackers to the individual command functions. The only requirement is adding tests to cover new scenarios where multiple trackers are supported. I have updated existing functions (namely _trackPageView and _trackEvent) that were using the previous copy-paste method to use the _gaMultipleTracker method instead.

[justinsa]

@revolunet - tests are 100% passing.

[Karpo72]

Ok looks nice...

[Karpo72]

When is the next version update planed?

[revolunet]

awesome thanks @justinsa ! will be released shortly

[revolunet]

released as 1.0.0 on npm. we should now follow semver