Added a code snippet for Vue

[xi-osman]

Added a code snippet on how a developer would initialize the FullStory SDK on a Vue Js app, This is also adding the SDK to the Vue instance property which will allow other Vue instances to have access to that property.

Example calling an event from another Vue component would look like this.

this.$FullStory.event("Subscribed", {
  uid_str: "750948353",
  plan_name_str: "Professional",
  plan_price_real: 299,
  plan_users_int: 10,
  days_in_trial_int: 42,
  feature_packs: ["MAPS", "DEV", "DATA"]
});

[patrick-fs]

Hi @xi-osman thanks for for the PR! We'll take a look.