fix to process from opcodes string, not just OpcodesInterfaces

Hi Rob, I'm playing around with decoding previous versions of articles stored as opcode-strings in a database, so I needed to be able to pass the opcodes-string as the second argument to <code>Render::process()</code>. Also, the example in the README shows this functionality, so I thought it would be a good idea to add a pull request on this.

PS. Something weird happened in the docblock (misspelled as 'codeblock' in the commit message), which is why I ended up doing 3 commits..