Allow specifying root selector/element for initial report. Fix #36

[jairovg]

General Notes

This PR allows the set-up of a root element different from the document to run the audit on the first load. Fix #36

NOTE: This element will be used as the default context sent to axe.run.

// Plugin setup
Vue.use(VueAxe, {
  element: '#app'
})

// manual a11y report using a different root element, document for this sample
this.$axe.run({
  clearConsole: true,
  element: document
})

What kind of change does this PR introduce? (check at least one)

• [x] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [x] Other, please describe:

Documentation updated with the new element for the plugin config.

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [x] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

[fugazi]

Good approach, this will enhance user experience.

[edward-montoya]

Awesome!!