Add support for raw extensions. This means that you can include the DER encoding

of an arbitrary extension: e.g. 1.3.4.5=critical,RAW:12:34:56 Using this
technique currently unsupported extensions can be generated if you know their
DER encoding. Even if the extension is supported in future the raw extension
will still work: that is the raw version can always be used even if it is a
supported extension.