Support accessing RegExp objects

[cwc]

Reference: http://izs.me/v8-docs/classv8_1_1RegExp.html

Currently when a RegExp is passed to Go from a JavaScript function call, it appears as an empty map (map[]).

It looks like you could get the string value (RegExp::GetSource()) and wrap that in a Go RegExp with MustCompile (http://godoc.org/regexp#MustCompile).

[mattn]

Added in f4cda62