strings: add Title

strings: add Title
strings.ToTitle converts all characters to title case, which for consistency with the
other To* functions it should continue to do.  This CL adds string.Title, which
does a proper title-casing of the string.
A similar function for package bytes will follow once this is settled.
Fixes #933.

R=rsc
CC=golang-dev
https://golang.org/cl/1869042