Extract sourceMappingURL from escaped string

[Kevan Davis]

Not sure how it happened, but getSourceMappingUrl is operating on an escaped string, so, the newline character is actually \\n instead of \n. I was able to fix locally by adding \\ to the regex on this line:

match = /^[^\r\n\\]+/.exec( substring );

but a better fix would be to unescape this string before matching