release 2018-02-01

Add a std::nullptr_t ctor overload to RE2::Arg.

This makes it possible to pass a C++11 nullptr as a capture argument
to RE2::FullMatch() and friends.

Change-Id: I34bdb1cd6b1e0670a262cebb0d7be8a4cec5c1fc
Reviewed-on: https://code-review.googlesource.com/22410
Reviewed-by: Paul Wankadia <junyer@google.com>