Document linkTarget

[adjenks]

I wanted links to open in a new tab and I found this blog: https://sosuke.com/writing-custom-extensions-for-the-remarkable-javascript-markdown-parser/

but I noticed that the source code has a options.linkTarget option so I didn't have to use it: https://github.com/jonschlinkert/remarkable/blob/dev/lib/rules.js

Please document all the options on the main page or put a link to a list of options.