feat(ios): make links work in iOS.

[ditoglez]

PR Checklist

• [x] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• [x] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.

What is the current behavior?

Links from markdown don't follow up

What is the new behavior?

Replaces UILabel with UITextView in order to allow the user to interact with the content coming from the markdown view. Also upgrades nativescript to v6

Fixes/Implements/Closes #3, #4 .