Can I create links with 'target="_blank"' in Markdown?

I don't think there is a markdown feature, although there may be other options available if you want to open links which point outside your own site automatically with JavaScript.

Array.from(javascript.links)
    .filter(link => link.hostname != window.location.hostname)
    .forEach(link => link.target = '_blank');

jsFiddle.

If you're using jQuery:

$(document.links).filter(function() {
    return this.hostname != window.location.hostname;
}).attr('target', '_blank');

jsFiddle.

Kramdown supports it. It's compatible with standard Markdown syntax, but has many extensions, too. You would use it like this:

[link](url){:target="_blank"}

With Markdown v2.5.2, you can use this:

[link](URL){:target="_blank"}

As far as the Markdown syntax is concerned, if you want to get that detailed, you'll just have to use HTML.

<a href="http://example.com/" target="_blank">Hello, world!</a>

Most Markdown engines I've seen allow plain old HTML, just for situations like this where a generic text markup system just won't cut it. (The StackOverflow engine, for example.) They then run the entire output through an HTML whitelist filter, regardless, since even a Markdown-only document can easily contain XSS attacks. As such, if you or your users want to create _blank links, then they probably still can.

If that's a feature you're going to be using often, it might make sense to create your own syntax, but it's generally not a vital feature. If I want to launch that link in a new window, I'll ctrl-click it myself, thanks.