This allows markup bracketed by punctuation even when the
punctuation has text on the other side, like in:
- Markup—_especially_italic_text_—can easily be overused.
- We want to increase `go`vet`'s usage.
Change-Id: I0c6ca790f23f705d8c8ba8a225c0280b916ebb6c
Reviewed-on: https://go-review.googlesource.com/33662
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>