Correctly handle link commands in multiline input.

Hey there!

I've been goofing around with gopherspace recently[1], and found your packages on npm. Nice stuff! I noticed that `:link` commands weren't rendering correctly, and traced it back to an annoying little typo, fixed by this PR. I also added a test to illustrate the bug.

Cheers,

Jared

[1] https://github.com/6a68/retrofuturisme