Add boolean prop, 'openInNewTab' to KExternalLink

[sairina]

Summary:

• Added boolean prop openInNewTab to KExternalLink
• Added :target to anchor tag in <template> to open up new tab if openInNewTab is given true

Fixes #94

Testing:

• Tested with Chrome on MacOSX, but not yet with other versions of Kolibri.
  

[indirectlylit]

This looks good, thanks!

It wasn't in the scope of the original issue, but I'd like to suggest that while we're doing this work, we include an iconographic representation of the "new tab" behavior. We do something similar in the design system itself:

thoughts?

[nucleogenesis]

@indirectlylit There is an issue for that #123 -

One thing not noted in that issue is that we want it to be conditional upon the new openInNewTab prop to be shown.

@sairina maybe we could add the icon. You'll find the name of the icon here: https://kolibri-design-system.netlify.app/icons/#icons that you'll want to pass to the :icon prop of <KIcon>

Additionally - we'll want to make sure that it displays with the correct color and on the right side of the link (just like in #123) - but when we're RTL it should show on the left side.

[nucleogenesis]

We'll want to also add rel="noopener noreferrer" attr to that <a> tag for security purposes

https://mathiasbynens.github.io/rel-noopener/ has a great breakdown of why we need to add this to external links.

[sairina]

@nucleogenesis Thanks for that explanation! I'll add that in.
@indirectlylit I'll work on adding the icon!