add target="_blank" to external links

[hanneskaeufler]

Whenever a link has a href to an external site, add the target to open it in a new window/tab. See issue #52

• [x] Add target to external links
• [x] Ignore when there is already a target set
• [x] Figure out why this doesnt work for the links in the "For example" section

  Example:

See link here which (ok it's confusing because it's unrendered markdown) does not have a target="_blank".

Will now render with target:

[orta]

Yeah @hanneskaeufler - this is looking 10/10

Wow, you're really picking this stuff up, I should start making issues for the things I want to get around to doing too 👯