language: added MatchString

It is very common to match a series of tags (which
may itself be sequences). This makes that easier.

Change-Id: I78b974b322a7d04c3226bae4c727d2911a992a4b
Reviewed-on: https://go-review.googlesource.com/58990
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>