add support for tabs in link body

[mikeando]

Should fix #191

[mikeando]

Ran this via github codespaces and npm test passed. Not sure if there's any other testing we should do.

Feels a bit like a test should be added somewhere for this behaviour - but I guess the right place for that is in the spec?

[jgm]

We try to avoid cluttering the spec with too many cases, so maybe the thing to do is to add a regression test. See test/regression.txt.