Backtick code in lists

This patch allows to include backtick code blocks into ordered lists without breaking numbering. There was an issue #488 about this but it was closed without resolving.

The idea is to replace each highlighted html with base64 encoded plain-text blob with same indentation. Blobs are surrounded by control UUIDs which help to find them back. For Markdown parser each blob looks like just a word so it doesn't touch anything, and after the processing is fully done, blobs are decoded back to html, fitting nicely into whatever structure (P or LI) has been generated around them.

Here are some test cases:

    ```m
    NSLog(@"simple top level");
    ```

    0. one
    BBBBB
    ```m
    NSLog(@"inside list, no indent");
    a <b> c;
    d && e;
    ```

    0. continue numbering
     BBB
     ```m
     NSLog(@"inside list, one-space indent");
     ```

    0. continue numbering
        BBB
        ```m
        NSLog(@"inside, list 4-space indent");
        ```

    0. continue numbering

        ```m
        NSLog(@"inside list, separate 4-space indented paragraph");
        ```

    0. continue numbering

    ```m
    NSLog(@"outside of list (must BREAK the numbering)");
    ```

    0. One again

I took a quick look for whether I can extend this for Liquid-based `{% codeblock %}`, but didn't find neither how to guess its initial indentation nor any kind of post-render event. Also nothing is done when `-raw` flag is present as I don't completely understand what it's for.