Make datalayer name dynamic for GA & GTM

[flashdesignory]

This makes dataLayerName required, but default values could be passed in from the wrappers.

example passing in 'GA-DataLayer' and 'GTM-DataLayer':

@kara

[housseindjirdeh]

Thanks for this. We should probably start doing proper semver versioning when this lands

[flashdesignory]

it'll be a minor release.. it's already set up with semver:

npm run release:minor

[huang-julien]

I think it would be a bit harder to correctly augment Window

How about accessing it through window.dataLayers[dataLayername] ?

[flashdesignory]

I think it would be a bit harder to correctly augment Window

How about accessing it through window.dataLayers[dataLayername] ?

Made the change, please review @huang-julien @housseindjirdeh