secure/precis: implementation of context rules

only lookback is supported. Lookahead (except for
the Katakana dot rule) is still forthcoming.

Change-Id: Idf285da486d954597563f987ba6e75fc99f5cb22
Reviewed-on: https://go-review.googlesource.com/23805
Reviewed-by: Nigel Tao <nigeltao@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
8 files changed