Support setting "user_properties" scope

[leo-petrucci]

I've noticed this library doesn't seem to support the very useful user_properties scope. Is there a reason why this isn't implemented or would you accept a PR to add it?

I was going to clone the source for my own project to add it, but it'd be far better if it was part of the library itself!

[MForMarlon]

@leo-petrucci You should be able to do it via ReactGA.set({ user_properties: { ... }});

[codler]

ReactGA.gtag('set', 'user_properties', {
  favorite_composer: 'Mahler',
  favorite_instrument: 'double bass',
  season_ticketholder: 'true'
});