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

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.