regexp: add Split

regexp: add Split

As discussed in issue 2672 and on golang-nuts, this CL adds a Split() method
to regexp. It is based on returning the "opposite" of FindAllString() so that
the returned substrings are everything not matched by the expression.

See: https://groups.google.com/forum/?fromgroups=#!topic/golang-nuts/xodBZh9Lh2E

Fixes #2762.

R=remyoudompheng, r, rsc
CC=golang-dev
https://golang.org/cl/6846048