Suggestion: support advanced mode without RegEx conversion

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Hi, I found it hard to test express-cors using a file based client because the origin is set to literal null (at least by Chrome), which is not a valid URL. Also, the pattern conversion you enforced make it hard to allow Any that includes above case. To kill two birds with one stone, I suggest providing an advancedMode option. When set, the input is assumed to be valid RegEx so no conversion is performed.