Added option to specify target attribute for link menus

[maldoinc]

While a very welcome feature the new link menus imo lack the ability to open them in a new tab. This is especially useful for external links where you do not want the user to leave your application.

While opening new tabs is still possible via scroll click, ctrl + click etc this patch makes it possible to make that the default behaviour.

A new target property can be set on the link menus which then will be passed on to the generated anchor tag.

Possible values: _blank|_self|_parent|_top|framename where _blank is by far the most useful one.

@javiereguiluz please tell me how to go about adding tests for this feature.

[Pierstoval]

I like the need for adding new features specific to each link, but I prefer having an attr option, like we have in form fields. This would be much more flexible, and plus it would allow adding classes or other cool things.

[javiereguiluz]

@maldoinc thanks for proposing this feature. I agree with it and for now I don't agree with @Pierstoval's proposal to generalize this and allow to define more options (let's add new options one by one).

I've created #907 to make some tweaks to your pull requests:

1. I've updated the tests, which is a bit tricky to do in our case.
2. I've removed the restriction that make this feature only work for link type items. Now it works for any menu item.
3. I've added a brief explanation in the docs about this new feature.

So I'm closing this pull request in favor of the newer one. In any case, I've reused your original commits, so you'll get full credit when merging this. Thanks!